Halachically whats the best time to have relations, early evening or later at night (Modesty as per the Rabbis)? Also if a couple are visiting friends or inlaws or are having guests over and its the mikvah night: Should relations be delayed for modesty purposes?
Halachah does not distinguish between different parts of the night, provided that it is after nightfall.
Based on kaballah, the best time is at or after chatzos (midnight according to Torah hours), on Friday night. Some equate Friday night before chatzos with a weekday after chatzos. However, this is not always convenient for the wife (or for the couple), and one must take care that the timing is optimal for the couple.
It is best not to delay going to the mikva. On the other hand, the Rema sees revealing the mikva night to others as a stringent matter (Yoreh De’ah, end of 198).
Although we find a precedent for postponing the tevilah in Mahari Asad (317), he refers to a case where the tevilah will become known to the entire town, and not to the ‘regular’ case of guests or of visiting one’s in-laws.
In this case, Shevet Halevi (Vol. 5, no. 118) writes that one should nonetheless go to the mikva, trying to hide the tevilah and to give excuses as best as possible. See also Teshuvos Vehanhagos 1:510.