What is cosidered the 1st day of a 28 menstrual cycle first your period or last?

The first day of a 28-day menstrual cycle is considered to be the first day of your period. This is because the menstrual cycle is counted from the first day of your period to the day before your next period begins. So, if your period starts on January 1st, your menstrual cycle will end on January 28th.