1,073. Establishing a Menstrual Pattern

153:16 The practice of women during the days of seeing blood until the time she wears white garments was not to go to shul, nor to daven. (Nowadays, this is not our practice - Mishnah Brurah 88:7. Even when people did this, it was not actually prohibited; they acted this way voluntarily - ibid. 88:6.) Even when women did follow this practice, they were lenient during the "Days of Awe," i.e., from the first day of selichos and on, if not going would disappoint her. Similarly, when she's carrying one of her children, or after having a baby when the time comes for her to go to shul (as will be discussed in chapter 158), and in similar circumstances, she is permitted to go.

154:1 A menstrual pattern is determined by three times in a row. That is to say, if a woman saw blood three times in succession at regular intervals, then this is considered the regular start of her period. Some women start their periods on the same day of each month: if she saw blood three times in a row on Rosh Chodesh, then her period is established as starting on Rosh Chodesh. Similarly, if she saw blood three times consecutively on the fifth day of the month, then the fifth of the month is established as the start of her period. This is called "veses hayamim" - a menstrual pattern based on days - because it starts on a given day of the month. This is true even though the number of days between the start of each period are not equal, as some months have 29 days and others have 30 days. Nevertheless, since she has learned to look on this day of the month, then this day is set as the start of her period.