There are two opinions as to when one should prepare for immersion on a weekday: during the daytime before mikveh night, so that one will not be rushed through preparations, or at night, to ensure one is clean and free of chatzitzot as close as possible to immersion. It is customary to follow both opinions when possible, by beginning preparations during the day and completing them at night.
However, when immersion is scheduled for Motzai Shabbat, and it is impossible to prepare for the tevilah during the daytime (on Shabbat), there are different opinions about when to do the preparations (chafifah). Some maintain that it is preferable to do the bulk of the preparations on Friday, so you will not rush through the chafifah. In this case, you should still shower and comb through your hair on Motzai Shabbat and visually inspect your body immediately before immersing. However, others maintain that it is preferable to do the preparations on Motzai Shabbat, so that the chafifah is as close as possible to the tevilah. In this case you must take care to spend half an hour to an hour on the preparations, so as not to rush through them. Even if you do your preparations on Motzai Shabbat, it is still recommended to do some of the more time-consuming preparations on Friday (cutting nails, shaving).
You should do the preparations and take the bath when it is most suitable for you, following the guidelines above. If you have more time on Friday, then do it then. If your Fridays are too hectic and it is more relaxed on Motzai Shabbat, then you can take the bath after Shabbat.
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