Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Menstrual Hygiene Day
May 28 has symbolic meaning. May is the 5th month of the year and women menstruate an average of 5 days every month. Also, the menstrual cycle averages 28 days.