The best way to try to determine when your
periods will come, and whether your
menstrual months are even (such as, always 26 days or always 28 days) or irregular (such as, sometimes 28 days, sometimes 30 days, sometimes 26 days) is to keep a calendar of your menstrual months. Either use an online app or just graph paper. Mark 1 through 30 or 31 across the top, and fill in the menstrual month dates below those numbers, beginning with the first day of your period.