We count the five days (and the seven days), according to the Jewish calendar. Thus, anything after sunset counts as the next day. Monday night after sunset is halachically considered Tuesday, so Tuesday is day one of the five-day minimum. Your seven clean days can begin no earlier than Sunday.

You can attempt a hefsek taharah on Shabbat afternoon, before sunset. If you stop bleeding before that, you could try an earlier hefsek as a back-up.

Please don’t hesitate to get back to us with any further questions.