Onah Beinonis: The 30th Day (and the 31st) Explained

5 min read · October 22, 2026

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The onah beinonis — the 'average' onah — is the veses based on the typical cycle of thirty days. It applies to every woman without a fixed pattern, and it is set afresh from each new period.

How to count it

The day the period began is day one. The onah beinonis is day 30. A period that began on 10 Cheshvan sets the onah beinonis on 9 Kislev in a month of 30 days — always count thirty days, not 'the same date', which is the veses hachodesh.

One onah or the whole day?

Strictly the veses is the onah in which the last period began. Many keep the full day — both the night and the day onah of day 30 — and this is a widespread custom. A calendar should mark the whole day unless your rav tells you to keep only the onah.

Day 31

The Chavas Daas holds the average cycle is reckoned as day 31, and many keep day 31 as well as day 30. Others keep day 30 alone. This is one of the settings your rav should decide.

A bedikah

A bedikah is made on the onah beinonis; many make it towards the end of the onah. If it was forgotten, ask your rav what to do before the couple resume.

When it no longer applies

A woman with a veses kavua — a fixed pattern — keeps her fixed veses, and opinions differ on whether she still keeps the onah beinonis alongside it. Long, regular cycles can also change what applies. These are questions for your rav, and a good calendar keeps both until told otherwise.

This guide explains the general rules and the common customs. Where poskim differ it says so — it does not rule. For your own questions, ask your rav.