Is it permissible for a wife who has been given a revocable divorce to delay her second menstrual period by using birth control pills in order to complete the fast of Ramadan, and is this menstrual period counted from the time it actually occurs or from its rightful time?

There is no harm in taking medication to delay menstruation as long as it does not cause harm, though it is preferable to avoid it. You are considered ritually pure when the bleeding stops, but the waiting period (iddah) does not end until three menstrual cycles have passed, calculated from the onset of bleeding. A woman who is revocably divorced should seek permission from her ex-husband before taking this medication, due to the harm it may cause him, such as prolonging the waiting period and the maintenance expenses.