Although it is ideal to have haircuts etc. on Fridays for Shabbat, is it permitted to have them any day of the week?
It is permitted to have a haircut on any day of the week, although it is preferred not to have a haircut on Thursday. The reason being that it takes 48 hours to notice that the cut hair is starting to grow back, and it isn’t the honor of Shabbos that it be noticeable that it is growing back then. If one doesn’t have a choice it is permitted to take a haircut on Thursday, such as when Rosh Chodesh is on a Friday for those people who don’t cut their hair on Rosh Chodesh.