In general, when one cannot prepare during the day on a particular occasion (e.g., Shabbat, Yom Tov), there is debate among authorities if it is better to prepare the day before or the evening of immersion. There is no clear cut answer. The issue is when you will be able to do the best unhurried job. You need to decide if this is before the fast (and then you take a quick shower and comb through your hair at the mikveh afterwards) or after the fast (remembering that you also need time to eat – remember to check if there is a change in mikveh hours that night).
It is also possible to take care of some things such as cutting nails on Monday to reduce preparation time after the fast. Women often find it difficult to prepare after the fast because they are tired and hungry, it is a late summer night, and twice as many women as usual will be using the mikveh (since there is no immersion on Tishah B'Av itself). Therefore, we recommend performing as much of the chafifah as possible on the day before the fast.
Often, the mikveh will remain open later the night after Tisha B'Av to allow one to eat before going to the mikveh. Just be sure to brush your teeth well after eating. If there is not time to eat at home, then pack something to take with you.
Preparation for the mikveh is not viewed as bathing for pleasure and therefore your usual preparation routine is permitted during the nine days (although not on Tisha B'Av itself). Concentrate on preparing and don't spend extra time luxuriating in the bath. There is debate among halachic authorities about married women shaving body hair during the three weeks, so it is best to check with your rabbi about this. As the rules of the nine days, including the prohibition of regular bathing, apply both Monday afternoon and Tuesday night, there is no advantage in delaying preparation to after the fast for this reason.