Onah Beinonit (The average onah)

There is a controversy among halachic authorities regarding the calculation of the onah beinonit.  Most authorities believe the onah beinonit falls on the thirtieth day from the beginning of a woman's last menstrual period. The first day of her period is counted as day one, and the onah beinonit is day thirty. The average halachic menstrual cycle is thus 29 days long.

Some authorities maintain that the average halachic menstrual cycle is thirty days long. According to this interpretation, the onah beinonit falls on the thirty-first day from the beginning of a woman's last period. Therefore, some women have the custom to observe both the thirtieth and thirty-first days as onot perishah.

A woman without a veset kavua observes the onah beinonit more strictly than other onot perishah. If she forgot to perform an internal examination on the onah beinonit, she must do one afterwards, and may not resume relations with her husband until she has done so. This is not required if she forgot to perform the examination on the yom hachodesh or on the haflagah. A woman with a veset kavua does not observe the onah beinonit.

Most onot perishah last for only one onah, either daytime or night-time. Some women, however, observe onah beinonit for a full 24 hours, from sunset to sunset, of whatever Hebrew date the onah beinonit falls on.

In the following example, a woman begins menstruating during the day on 15 Nissan. She counts 15 Nissan as Day 1, and her onah beinonit falls (according to the standard calculation) during the day on 14 Iyar, Day 30.

Nissan

Rishon	Sheini	Shlishi	Revi'i	Chamishi	Shishi	Shabbat
1	2	3	4	5	6	7
8	9	10	11	12	13	14
15 (1)	16 (2)	17 (3)	18 (4)	19 (5)	20 (6)	21 (7)
22 (8)	23 (9)	24 (10)	25 (11)	26 (12)	27 (13)	28 (14)
29 (15)	30 (16)

Iyar

Rishon	Sheini	Shlishi	Revi'i	Chamishi	Shishi	Shabbat
		1 (17)	2 (18)	3 (19)	4 (20)	5 (21)
6 (22)	7 (23)	8 (24)	9 (25)	10 (26)	11 (27)	12 (28)
13 (29)	14 (30)	15	16	17	18	19
20	21	22	23	24	25	26
27	28	29