A. When is Sex Forbidden?
Islam has forbidden sexual intercourse during menstruation.
The Qur'an says: They ask you about menstruation. Say: “Menstruation is a discomfort (for women). Do not establish sexual relations with them during the menses and do not approach them (sexually) until the blood stops. Then when they have cleansed themselves, you go into them as Allah has commanded you.” 7
According to the Shariah, the duration of the monthly period is between three to ten days. If the bleeding was for less than three days, it is not menstruation; if it is for more than ten days, then it is menstruation for the regular number of days and istehadha for the rest of the bleeding during which sex is permitted.
The prohibition of sex during the periods is limited strictly to sexual intercourse; other intimate contact (with the exception of the vagina and anus) is allowed. However, it is better not to play with her body between the navel and the knees.
If a person who is engaged in sexual intercourse with his wife discovers that her period has begun, then he should immediately withdraw from her.
It is clear from the verse mentioned above (until the blood stops) that once the blood has stopped, intercourse becomes lawful even if the woman has not performed the major ritual ablution (ghusl). But mujtahids say that it is better to refrain from intercourse till she performs the ghusl or, at least, washes her private parts.
Sexual intercourse is also not allowed during the post-natal bleeding called nifas (maximum 10 days), during daytime in the month of Ramadhan, and when a person is in ihram during the pilgrimage to Mecca. At all other times, sexual intercourse is allowed.
Times when Sexual Intercourse is makruh:
i. During frightful natural occurrences, e.g., eclipse, hurricane, earthquake;
ii. From sunset till maghrib;
ii. From dawn till sunrise;
iii. The last three nights of lunar months;
iv. Eve of the 15th of every lunar month;
v. Eve of 10th Zil-hijjah;
vii. After becoming junub.
B. Recommended Days and Times for Sex
We have certain ahadith which say that it is better to have sexual intercourse at these times:
i. Sunday night;
ii. Monday night;
iii. Wednesday night;
iv. Thursday noon;
v. Thursday night;
vi. Friday evening;
vii. Whenever the wife wants to have sex.
C. When is it Obligatory to have Sex?
It is wajib on man to have sex with his wife at least once in every four months; this is considered as one of the conjugal rights of the wife. This obligation stays in force unless there is a valid excuse or the wife waives her right.
 Wasaelush Shia, vol. 1, p. 576