Importance of Garbhadhana Samskara:
If one is not purified by the process of the seed giving ceremony, or garbhadhana samskara he is immediately classified amongst the sudras because only the sudras do not undergo this purificatory process.
Krsna book, IIp.236
The pious seminal succession in the twice born families of the brahmanas and ksatriyas especially, as well as in the families of vaisyas also, must be kept very pure by the observation of the purificatory process (samksaras) beginning with garbhadhana samskara, which is observed before conceiving a child. Unless this purificatory process is strictly observed, especially by the brahmanas, the family descendents become impure, and gradually sinful activities become visible in the family.
SB 4.14.42, purport
It is to be concluded that while conceiving a child one's mind must be very sober and devotional. For this purpose the garbhadhana samskara is recommended in the Vedic scriptures.
After the marriage the newly wedded couple should observe brahmacarya for three days, sleeping on the earth. On the fourth day, the couple go to the husband's house and caturthi homa is performed. After this, during the day time, the preliminaries for the first garbhadhana samskara may be performed, taking into consideration the suitable and forbidden times . It may also be performed after six or twelve nights, or after one year. The rules for time and the samskara should be observed thereafter whenever the desire for sex and children arises. To say the least, each sex act must be carefully planned in advance, by use of intelligence, using the scriptural injunctions, and not determined by lust alone.
...Before engaging in sexual intercourse, both the husband and the wife must consider their mental condition, the particular time, the husband's direction and obedience to the demigods. SB 3.14.38, purport
The union of man and wife should take place neither during the day nor during the sandhyas (daybreak and twilight). Daytime is of course the time when man should carry out his duties: waking bathing, meditating, worshipping the Supreme Lord, studying scripture, chanting japa, serving the deity, carrying out household duties, welcoming guests etc. Sandhyas should only be utilized for chanting gayatri mantra and meditating on the Lord. As well, Prasnopanisad says that a man reduces his life span by having sexual relations during the day. One who performs during the night is considered to be brahmacari (ie he maitains his strength).
pranam va ete skandanti ye diva ratya samyujyante
brahmacaryam eva tad ya ratrau ratya samyujyante
According to Asvalayana Smrti from the end of the evening sandhya up until midnight is allowed:
Exception to this is when the wife is extremely lusty. To prevent her from going elsewhere for satisfaction the husband should concede to her demands, though not according the scriptural recommendations of proper time.
Numbering the nights from the start of the menses of the woman, then at least the first four nights are forbidden for sexual activity. Kalaprakasika, states that the first day is fatal for the husband,the second day is fatal forthe wife, thethird day causes abortion and the fourth day impairs the power of the father and produces a short lived son bereft of good habits.
Srila Prabhupad gives a slightly longer period.
The recommended period is six days after the mensturation period.
letter from Srila Prabhupada,
Shama dasi, Jan.18, 1969
The eleventh and thirteenth nights are also forbidden, as the offspring produced are of bad quality. The remaining ten days are permitted, and the later nights are favored. Even numbered nights favor male children, and uneven numbers favor female children. Sexual union after the sixteenth night is also forbidden. In the sequence below the starred days starting from 1 (beginning of menses) are forbidden for sexual intercourse.
continue in the next blog,s