Menstrual cycles can vary however, it is not uncommon for a person's cycle to come a few days early or late. However, if you are of childbearing years and sexually active and your period is more than a few days late, you should probably go to the doctor to determine whether or not pregnancy is an explanation for the delayed period.
Even if the girl is normally pretty regular, it can still be consider normal to be a little late with her period. You have to take into consideration stress, illness, body changes, and even climate changes. Any of those things can cause a period to be several days to a week late.