According to the Sefer Yetzirah (Mishna 1:5), the space continuum consists of three dimensions and the continuum of time, of two directions (past and future). As such, the Universe is constituted by four dimensions. Moreover, the Sefer Yetzirah explains that there is a fifth dimension, to wit, the moral dimension (spiritual, or soul), which possesses two directions: good and evil. This moral dimension underlines all other dimensions from the space-time continuum. The spiritual trajectory towards good or evil depends on the actions that man impresses upon the dimensions of the Universe, which reverberate in all of the olamot (spiritual worlds and this one, physical). For instance, we see this occurring in the procreative act. As it is known, “if the person sanctifies himself [and has the appropriate kavanot], he will bring a holy garment for the neshama of his child [which will be conceived through the act], and in this way, he will enable him to serve Hashem promptly. Regardless of the greatness that this soul may attain, it is nevertheless important that the father [and the mother] be sanctified during the act. But the neshama itself, apart from its ‘garments’ [i.e., the power of thought, speech, and action which clothe the neshama in its particular spiritual root and degree, which is solely decided by Hashem], is not affected by the sanctification [and kavanot] from the parents; in fact it happens at times that, the soul of a person infinitely elevated comes down and becomes the child of an ignorant and lowly person” (Tanya, Likkutei Amarim 2). However, the ‘spiritual replies’ to the actions of man on the physical level are also bound (and thus dependent) by time and its cycles, which albeit intimately connected to the physical continuum, do transcend it. This is the spiritual law of סוף מעשה במחשבה תחלה Sof ma’aseh, b’machshava tehilah “last to be created, first to be conceived”, alluding to the Bnei Israel, which establishes the precedence of time over space, regardless of its dimensional connection. Hence, certain positive actions, even when appropriate but fulfilled on ‘inappropriate times’ can be without any effect or worse yet, with the contrary effect, chaz v’shalom. For example, Kabbalah Ma’asit teaches that kameyot/kosher amulets for different purposes (e.g., protection for pregnancy, see Ta’ameiHa’Minhagim; Toldot Adam; R’Menashe Ben Israel, Nishmat Chayim 3:25; Midrash Talpiyot 1, Efods.v.V’HaEven; Deguel Machane Efraim, Vayicra, et. al.) need necessarily be written not only by someone with extraordinary knowledge and purity, but in precise times, that is, on specific months, days, and even hours of the day! The writing of the kameyot, which disrespects the correct times, loses its positive value, which comes from its alignment with the intermediary angelical forces that govern the five dimensions, thus rendering it incomplete (i.e., without the influence on the angelical entity) and even dangerous (i.e., by calling the attention of negative entities, G-d forbid).
And this is exactly how it was for Yosef. He knew the secrets of the illumination from kedusha/holiness called the time of “seasons, for days and years” (Bereshit 1:14) of plenitude. But he also knew well the secrets “illumination” from tumah/impurity, called the time of time of ‘seasons, for days and years’ of famine. For to know the secrets of kedusha is to know the secrets of tumah. And what is this secret of tumah? “The secret of not procreating [when the sitra achra is ‘illuminated’, to wit] on the ‘seasons, for days and years’ of famine… When the time of famine reigns, it behooves man to sexually restrict and not to allow that the sitra achra be ‘multiplied’ [through the strange souls that in this time of famine descent to the children to be] in the world, G-d forbid” (Zohar 204a, Miketz). In fact, the Ramak (R’ Moshe Cordovero) explains that, “The husband’s marital union with his wife should occur only when the Shechinah is in Its place, which means, between the ‘two arms’ [Chesed and Gevurah]. Now, when the Community is in danger [of famine] and the Shechinah is not between the two arms, these unions are prohibited” (Sefer Tomer Devorah, Perek Tet, citing the Tikkunei HaZohar from parashat Bereshit, Tikkun 69). It is for this reason that Yosef HaTzaddik made sure to procreate ‘before’ this bitter time, thus avoiding any admixture in his acts – through the use of the holy and creative power of his brit kodesh – with the sitra achra. In this manner, he truly prevented the increase of the domain of profanity that would have benefited otherwise, chazv’shalom, from the kedusha.
(Note: it is equally not a proper time to have marital unions on a night preceding a tzom/fast save exceptions).
The Kabbalah asks: “In what are human beings distinguished from the horse and the mule? By sanctification and self-perfection. Hence the marital intercourse of human beings should be at fixed times, that they may concentrate their thoughts [of Torah] on cleaving to the Holy One, blessed be He” (Zohar 49b, Tazria) with the kavanah of V’yehi noam Hashem Elokeinu aleinu u’ma’aseh yodeinu konenah aleinu, um’aaseh yadseinu koneneihu, “May the pleasantness of Hashem our G-d be upon us, and may the work of our hands prosper” (Tehillim 90:17). And what is the time of the day that they may have marital union? Our Sages answer that, “It is the derech eretz [proper conduct] that intercourse should be at night” (Talmud, Ketubot 56a). And regarding the frequency, it is known that ‘man of faith’ have marital union with their wives “all Fridays, on the Shabbat nights” (Ibid. 62b; Orach Chayim 280:1) – certainly a great mitzvah. Note that, although thehalachah permits relations occuring on the other nights, ours Sages insist in the rigour of fulfilling this mitzvah only on the holy nights of Shabbat (see R’ Eliyahu de Vidas, Reshit Chochmah, Sha’ar HaKedusha 16) and on the night of the woman’s immersion on the mikveh etc., unless this frequency is not sufficient in order to avoid machshavot zarot and equally strange behaviors (see Kitvei Ari”zal).
Now, why is it so important to fulfill this mitzvah on the nights of Shabbat? In fact, there are great mysteries regarding the origin of zivugim (holy unions) on Shabbat, the time when the Holy One unites with the Shechinah. These celestial unions cause blessings to descent from above in order to nourish the days of the week. Indeed, they are put into effect both on the spiritual and physical domains. Kabbalah explains that, on each and every Shabbat night “occur zivugim which are initiated [and awakened] by the malachim (angels). Through these zivugim, spiritual answers of beneficence from the levels above them are elicited… Therefore, we are allowed to engage in conjugal relations on that particular night” (the Ari”zal, Sha’ar HaPesukim, Pinchas). The reason these zivugim are awakened by malachim is the manner Hashem has established the Divine plan through which His blessings are channeled and thus brought to all of Creation – including the more inferior degrees (i.e., our physical world). Marital unions on the night of Shabbat emulate the zivugim (occurring on the Partzufim) that are awakened by the angels. Hence, the vital importance of fulfilling this mitzvah in the exact manner described – to bring great blessings to the couple and the world at large. However, even though it is still Shabbat, the daytime (of Shabbat) is not permitted for marital unions, for during the day, “we need to awaken the celestial zivugim which elicit a flux of neshamot [to the world]” (ibid. Ari”zal). This means that, at night, our marital unions have the power to emulate the celestial zivugim, which are already occurring, but we cannot initiate them. We can channel the shefa (of blessings) through this emulation of the malachim while it occurs on Shabbat at night. During the period of Shabbat day, we are the ones that need to awake the zivugim through our avodat Hashem (e.g., in our study of Torah, holy seudah, etc.) – for presumably in this period of the day, the angels do not do any awakening! On the other side, when we put into effect these unions, we cannot channel them, for in order to do so we would need to stop the (described) holy activities to then engage in marital unions (which when emulate the zivugim of the angels, can truly channel down the shefa). This would leave us without anything to channel, to wit, no blessing would result from the conjugal act. This is obviously contrary to the described kavanah and this mitzvah’s purpose of procreation, as we would be left out of blessings, chaz v’shalom.
The Talmud explains (see Nedarim 20a-b) that, ‘different actions’ during the marital intercourse determine important physical and spiritual aspects from the soul that will, G-d willing, descent for the child’s body. The TannaRabi Yohanan ben Dahabai – who mystically explains having received knowledge on these subjects through the agency of angels – says in this tractate that, in accordance of “certain” sexual acts, children may be born “dumb, deaf, or blind”! Moreover, there are also effects caused strictly through the ‘time’ when the union occurs. Thus, Kabbalah affirms that: “Truly, there is that which is a ‘proper time’ for marital intercourse… and no other time is appropriate for this junction to be performed with all joy without any ‘extraneous admixture’ [i.e., the chitzonim] save that night [i.e., Shabbat. See Talmud, Ketubot 62b] – when the souls are distributed to the wise, the righteous, and the pious” (Zohar 204b, Vaya’khel). We see then that actions and distinct times in the fulfillment of the mitzvah (of marital union) can afect the soul that is to come. Moreover, through kavanot, the parents may spiritually elevate themselves to infinite heights and greatly impact this soul!
Regarding the subject that the ‘the parents may spiritually elevate themselves etc.’, as it is known, the ‘sexual energy’ (the physical form of the spiritual energy) is an extremely pure energy and must be care for and not wasted due to its fundamental role. As it has been explained, it allows – in accordance to the degree of purity it is used – great ascensions or great falls, chaz v’shalom. As it is written: Vehineh sulam mutsav artsah verosho magia hashamaymah vehineh mal’achei Elokim olim veyoredim bo, “Behold a ladder set up on the earth, and the top of it reached to heaven; and behold the angels of G-d ascending and descending on it” (Bereshit 28:12). On this pasuk, Kabbalah explains (see Zohar 149b, Vayetze) that, “The ladder signifies the spiritual attribute of Yesod [connected to sexual subjects]. To Ya’acov avinu, it was shown that this attribute exists on earth – it literally belongs to ‘mundane subjects’, however, the ‘top of it reached to heaven’ – to wit, when the person heeds to the Divine will and raises this attribute to its origin, then he truly reaches heaven… and ‘this is alluded in thesecret of the pasuk, Mi ya’aleh lanu hashamaymah, Who shall go up for us to heaven [Devarim 30:12]. The initial letters, the Reshei Tevot of this, spell out the word milah [circumcision], and the final letters, the Sofei Tevot, spell out Havaya’ [Tikkunei Zohar 2a] – this means that the person can ascend on high if he purifies this attribute” (the Slonnimer Rebbe, Netivot Shalom).
Regarding the subject of ‘greatly impact this soul’, beyond of has been already considered, it is important to comprehend something of the kavanot during the sexual union. It is very important that these unions be fulfilled in complete joy and happiness. Every mitzvah must be done with great joy, but this one, even more so. It is also important that the married couple, “both should have the same kavanah” (Zohar 81a, Kedoshim; see ibid. 83b for the contrary): to bring down to this level a noble soul, with yirat shamayim (fear of Heaven), a soul that will illuminate the world, to wit, a tzaddik. Hence, it is good to bring to mind a tzaddik on the moment of procreation (see Ramban, Iggeret HaKodesh; R’ Eliyahu de Vidas, Reshit Chochmah, Sha’ar HaKedusha 16). Furthermore, “it is necessary to know that, a child born from the predominant desire from the husband [to have a child, as have been described], receives an ‘extra’ vitality from the father, for this desire means the father’s wish ‘to connect to the Etz Chayim’” (Zohar 209a, Vayigash) in the secret of “She is a Etz Chayim to those who lay hold on her; and happy is every one who holds her fast” (Mishlei 3:18). It is also known that, “‘If the woman emits her semen first she bears a male child; if the man emits his semen first she bears a female child’ [Talmud, Niddah 31a]… Now, he who emits seed last is the one who prevails [i.e., something of his strength]… so the man [who wishes to have a male child] becomes cold, so as to not become heated [during the act], and in this manner the woman is heated [giving seed] before. Afterwards, he is the last to emit seed [thus ensuring that child’s vitality, as explained]. However, many who have followed this did not succed, for all is [baruch Hashem, and thus,] in the Divine will” (RabiYehudah Fatiyah, Minchat Yehudah, Parashat Shmuel I, Siman Bet, Ayin Tet, pg. 118). In any case, the person must sanctify himself at the time of marital relations, for all comes from merit. And let him pray throughout the first 40 days of conception: Yehi ra’avah mi’maarei alma shenizke lebanim yierim ushlemim va’chareidim lidvar Hashem venire’eh bachem kedushat tanaim ve’amoraim amen, “May it be the will of the Master of the Universe that we merit G-d fearing and flawless sons who tremble at the word of Hashem. And may we observe in them the sanctity of the Sages of the Mishnah and Talmud. Amen” (Rabi Tzvi Hirsch Kaidenover, Kav HaYashar, Perek Yud Zayin).