How to Calculate the Vestos: Onah Beinonis, Veses HaChodesh and Haflagah

8 min read · October 22, 2026

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The vestos (singular veses) are the days on which a woman anticipates her next period. On each, the couple keeps apart for the onah — perishah — and a bedikah is made. For a woman without a fixed pattern there are three, all counted from the onset of her last period, and all kept at the same onah (night or day) the period began in.

1. Onah beinonis — the 30th day

Count the day the period began as day one; the onah beinonis falls on day 30. Many keep the whole 24 hours of day 30, night and day, rather than only the onah the period began in. Many also keep day 31 (the view of the Chavas Daas), so day 30 and day 31 are both observed.

2. Veses hachodesh — the same Hebrew date

The veses hachodesh is the same Hebrew date in the next month, at the same onah. A period that began on the night of 15 Cheshvan sets a veses hachodesh on the night of 15 Kislev.

When the period began on the 30th and next month has only 29 days, customs differ: many keep the 1st of the following month, others the 29th, and some keep nothing that month. Your calendar should let you choose.

3. Haflagah — the interval

The haflagah is the interval between the last two periods, counted again from the most recent one. Count from the onset of the previous period to the onset of this one, counting both days; the haflagah falls on that same day-count after this onset, at the onah this period began in.

Two customs change the arithmetic. Some count the interval in onos rather than days (the view of the Chazon Ish). Some — Chabad among them — count from the hefsek taharah rather than from the onset. And when a shorter interval follows a longer one, many continue to keep the longer interval as well until it has passed without a period.

A worked example

Her previous period began on the day of 3 Tishrei; this one began on the night of 1 Cheshvan, 29 days later counting both days. She keeps: the onah beinonis on day 30 (and day 31 if she keeps the Chavas Daas); the veses hachodesh on the night of 1 Kislev; and the haflagah on the night 29 days on, counted from 1 Cheshvan as day one.

Or zarua

Many also keep the onah before each veses — the or zarua — so a night veses is preceded by keeping the day before it. Whether, and for which vestos, is a matter of custom.

Why a calculator helps

None of this is hard once, but it is easy to get wrong month after month: months of 29 and 30 days, a period that falls on bein hashmashos, an interval in onos, a veses uprooted by a later period. A vestos calculator that works in Hebrew dates and onos — and lets your rav set the opinions — removes the arithmetic and leaves you the decisions.

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.