Nishmat's Women’s Health and HalachaIn memory of Chaya Mirel bat R' Avraham

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Day/night calculations

20 May, 2005


Hi! Could you please tell me the easiest way to teach Onot when they switch from day to night?

Here is my example:

Let's say:

A woman got her period on the 20th day of Tishrei during the day. The next time she got her period on the 21st of Cheshvan at night. Could you please give me the Onot, and tell me the easiest way to teach to a Kallah?


All your onot perishah will be observed during the same onah (day or night) as the onset of your most recent period. So if your most recent period began at night, all three onot perishah will fall at night.

  • Yom hachodesh is easy: turn to the next month in the calendar, and mark off the same date and the same onah (day or night) as the beginning of your most recent flow. That would be 21 Kislev at night.
  • For the haflagah, you need to count the number of days from your previous period to your current one, and then count forward the same number of days. It doesn't matter how you do this as long as you are consistent. After you have calculated the date, mark off the correct onah (day or night, as in the beginning of your most recent flow). In this case, the interval is 32 days and comes out on 23 Kislev at night.
  • Onah beinonit is Day 30 of your cycle (the day you get your period is Day 1). This is always four weeks and one day from the start of your period. So if you got your period on a Thursday, onah beinonit will fall on a Friday four weeks later. Onah beinonit is always either the same as yom hachodesh (after a 29-day month), or one day earlier (after a 30-day month). After you have calculated the date, mark off the correct onah (day or night, based on whether you began your most recent flow in the day or night). Here, this comes out on 21 Kislev at night.

The above is the standard procedure in most communities. If you are teaching according to Chabad minhag, please let us know.

Please feel free to write back with any additional questions.

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