Periods change dates because the average menstrual cycle varies from 24 to 35 days. This basically means that a woman will start her period between 24 and 35 days after the beginning of the last one.
A woman does not start her period on the same day of each month. The cycle means that she will likely start on a different day each month and the projected start date will shift along with the cycle.
Many women have fluctuating cycles. This means that they might have a cycle that lasts for 24 days on one month and 35 days the next. This can be frustrating and confusing for many young women, but it does tend to become more dependable over time.
Because of the cycle, young women are advised to keep a log of their period start and end dates. This will help them determine the length of their cycle.
The important thing to remember is that every woman is different, and it is very common to experience irregular periods too. If you have any concerns, the best thing to do is to speak to a trusted person such as a parent, teacher or doctor.