Can you have relations the night before haflaga?
There is an opinion of one of the Rishonim, called the Ohr Zarua, that says that a couple should abstain from having relation during the “onah” that preceeeds the haflaga. Meaning that if the onah is during the day, that they should abstain the night before, The Ohr Zarua is not brought in the Shulchan Aruch, however numerous poskim say that prefferably it should be kept. If there is a need for a mitzvah, such as the night of the mikva, the night before the husband travels, according to some poskim during shana rishona, etc. then it is surely permitted.
If it is the night of the 30th day then there are even more opinions that one should be careful, because we keep the whole 24 hour period of the 30th day.
Yora Deah 184-2, Taz Ibid-2, Shach 7, R’ Akiva Eiger on Shach, Aruch Hashulchan, Chochmas Adom 105-3, Kitzur Shulchan Aruch 153-2, Igros Moshe Y:D 3-48, Be’er Moshe 3-137, Shiurei Shevet Halevi, Levushei Oz, Mareh Cohen pg. 170-4,