My husband is away on business & I am due to go to mikvah in the middle of his time away. Should I be going or should it be postponed to the night before his daytime arrival home or to the night after his arrival?
The custom is that women do not go to the mikva when their husband is not in town.
It should therefore be postponed until the night before/after his arrival, depending on need. If there is a preference of going the night before he arrives, to be tehorah when he comes, this is permitted. Some write that in this case, an item of his clothing should be placed on the bed.
The primary sources for this question are in Yoreh De’ah 197. The Beis Yosef cites opinions as to whether we say that tevillah bizmanah is a mitzvah or not, and the Shach (197:3) and others explain that le-halachah we maintain that tevillah bizmanah is not a mitzvah.
Because of this, it is not permitted to a woman to go to the mikva on Shabbos when her husband is out of town.
Later authorities explain that even during the week, a woman should not go to the mikva when her husband is absent, becasue of Ruach Ra’ah. See (for instance) Ben Ish Chai, Shanah Shniah, Shmini 20.