Menstrual cycle is counted from the first day of period to the first day of the next period. For most women the menstrual cycle is between 21 to 35 days.

Illustration of phases of menstrual cycle showing that the first day of the cycle starts from the first day of menstruation - Menstrupedia
References:
  1. Menstrual cycle: What's normal, what's not, http://www.mayoclinic.com