The menstrual cycle, which is measured from the first day of one period to the first day of the next, differs from woman to woman. Menstruation can happen every 21 to 35 days and last two to seven days. Long cycles are frequent in the first several years after menstruation begins. If you don't identify a pattern after four to six months, see your doctor. No, but it's important to note that the calendar date will change if there is a holiday, weekend, or any other type of interruption to the regular schedule. For example, if you regularly start your periods on Monday, January 1st, then the 25th of December will be a Friday, so when you think about your life before and after the holiday season, you'll know what date to expect in the future. Menstrual cycles do not affect the time it takes for Earth to orbit the sun. This process is called "the elliptical orbit." The moon's influence is only seen during a part of the month known as "lunar phase." When the moon is full, it covers exactly half of Earth's surface, leaving half dark and half light. As the moon moves away from fullness, more of Earth shows at once, so that by the time it reaches its most distant point from us, all of Earth is illuminated. Lunar phase has many effects on women's bodies.