You've actually already answered your question. If you have a 25 day cycle then your period will start on day 1 and occur every 25 days!
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From the day it starts.
The menstrual cycle begins with day 1 which is the first day of bleeding. The last day of the cycle is the day before the next period begins.
The first day of a womans menstrual cycle is also the first day of menstruation.
Ovulation generally occurs between day 11 and day 21 of a woman's cycle.
Day 1 is the first day of bleeding. Last day is the day prior to the start of next period. Average cycle is 28 days with an average bleeding duration of 3-5 days.