Periods need not come on a fixed date. Upto 7 days variation here and there is acceptable.
Make a note of your cycle length by calculating from the first day of your period to the first day of the next period cycle. If it doesn't vary more than 7 days then it is normal.
If it is more than that, you will need evaluation.
Health Tips
Recent changes in body weight and stress can also cause period irregularities. Maintain a good diet and exercise regimen to regularize your periods.