Sexual union is best practiced at night, or at dawn, because the acts of creation always take place in the darkness. The cosmic influence of the sun prohibits the creation of solar bodies. Nevertheless, daytime practice is acceptable on occasions in which other times are impossible, just be aware that only the comprehension and elimination of the ego can take place during daytime practice.

"And it came to pass, when he had been there a long time...with Rivkah his wife" (Beresheet 26:8): IT IS SAID "with (Heb. et) Rivkah his wife," precisely, which alludes to the Shechinah that was with Rivkah, BECAUSE ET ('WITH'), AS WE KNOW, IS THE NAME OF THE SHECHINAH. Another explanation asks if we could possibly conceive of Yitzchak performing his marital duties during the daytime. We have learned that the children of Yisrael are holy and abstain from cohabitation in the daytime. Therefore, how could Yitzchak, who was holy, cohabit during the day? - Zohar 4:12:97

Sexual transmutation between man and woman must never be practiced more than once per day, due to what is called the Creative-Magnetic Pause. The sexual energy cycles through the body every 24 hours, and after an act of sexual magic this energy is heavily magnetized. Thus, to repeat the act again in the same day disturbs the natural rhythms of the body and ultimately nullifies the great potential of the sexual energy, or even worse, transforms it into something harmful.

However, prayanama can be practiced as much as one needs. Many students find that pranayama is useful as an accompaniment to performing other tasks.

The energy of the Third Logos must be worked with only at night.  The work in the Laboratorium of the Holy Spirit must be performed during the nocturnal hours.  The Sahaja Maithuna can only be practiced in the darkness of the night, because during the day the Sun is opposed to any generation.

If one puts a hen with her eggs in the light of the sun in order to incubate them, then these eggs will not attain the incubation, and if some chickens hatch, they will die, because the sun is an enemy of generation.

Whosoever wants to search for the light must ask for it in the profound night, asking the Logos who is behind the Sun which illuminates us.

The crude reality is that because of the disposition of the creative organs, procreation is always achieved in darkness, because when the sperm exits the sexual glands, it does not exit illuminated by the light of the sun, but in darkness.  In darkness it opens its way through the fallopian tube in order to encounter the ovum which descends from the ovaries, and within the darkness of the womb, gestation is achieved.

But if this sperm, instead of exiting the sexual glands protected by the darkness, could exit under the light of the sun, and if the fetus was not in darkness and was uncovered in the womb of the woman in order for the sun to directly enter it, then it is obvious that failure would be a fact.

Therefore, fecundation is performed always in darkness, because of the disposition of the very organs of nature.  This is why, in order for one day to attain the intimate realization of the Being, the work must be performed within the obscurity of silence and within the august secret of the Wise.  This is what the Virgin of Immaculate Conception, who is standing upon the moon and dressed with a blue tunic, is indicating unto us.  The work of the Maithuna is performed within the darkness of night.

We must warn that the practice must never be performed two successive times in the same night.  The practice is allowed only one time a day.

When Sexual Magic is practiced two successive times, then violence against nature exists, because the law of the creative magnetic pause is violated.

It is also urgent to know that the spouse must never be obligated by force to practice Maithuna when she is sick or with her menstruation, or when she is pregnant, because these are crimes of violence against nature.

The woman who has given birth to a child can only practice the Maithuna forty days after childbirth.

There also exists the crime of violence against nature when the man or the woman forces their mate to perform the copulation when the organism of the spouse is not found in the suitable conditions for intercourse.

There also exists such a crime when the man or the woman obligate themselves to perform copulation even with the best intentions of realization of the Self and with the pretext of performing Sexual Magic, when in reality the creative organs are not found in the precise amorous moment and in the favorable harmonious conditions which are indispensable for copulation. - Samael Aun Weor, The Initiatic Path in the Arcana of Tarot and Kabbalah