Manusmriti 3.45.
Let (the husband) approach his wife in due season, being constantly satisfied with her (alone); he may also, being intent on pleasing her, approach her with a desire for conjugal union (on any day) excepting the Parvans.

3.46. Sixteen (days and) nights (in each month), including four days which differ from the rest and are censured by the virtuous, (are called) the natural season of women.

3.47. But among these the first four, the eleventh and the thirteenth are (declared to be) forbidden; the remaining nights are recommended.

3.48. On the even nights sons are conceived and daughters on the uneven ones; hence a man who desires to have sons should approach his wife in due season on the even (nights).

3.50. He who avoids women on the six forbidden nights and on eight others, is (equal in chastity to) a student, in whichever order he may live.