If you are asking about ghusl e haydh in the month of Ramadan then if you finish your periods before fajr then you must take ghusl before fajr and fast that day and if there is a reason you can't take ghusl then tayammum.

For example you finish your periods at 9 in the night, then you must take your ghusl before the fast of the next day (and also pray maghrib and Isha)

If you finish your periods after fajr, then you take ghusl and fast the next day. For example you finish your ghusl at 10 in the morning, then you must take ghusl for the next Namaz and then fast the following day.