The Taharah Calendar: A Complete Guide to Niddah, Vestos and Mikvah Night

7 min read · October 22, 2026

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A taharah calendar — a luach, a niddah calendar, a vestos calendar — is the record a married woman keeps so that she knows two things at all times: which days she must keep apart from her husband in anticipation of her period, and, once a period has begun, when she can make the hefsek taharah, count the seven clean days and go to the mikvah.

It looks like an ordinary calendar, but it runs on different clocks. Everything is counted by Hebrew date, and every Hebrew date is split into two halves — an onah of night and an onah of day.

Hebrew dates and the two onos

A Hebrew day begins at night. The night onah runs from sunset until sunrise, and the day onah from sunrise until sunset. So a period that begins on Tuesday at 9 in the evening began on the night of Wednesday's Hebrew date, and the vestos that follow from it are counted from that night onah, not from Tuesday.

This is why the exact time matters, and why a time close to sunset needs care. Between sunset and the appearance of the stars — bein hashmashos — it is unclear whether it is still day or already night, and customs differ on how such a moment is counted. A good calendar lets you record the time and the place, and works out the onah from that day's sunset where you were.

What the calendar tracks

• The onset (re'iyah): the date, time and onah a period began.
• The vestos: the days on which the next period is anticipated — the onah beinonis (30th day), the veses hachodesh (same Hebrew date next month) and the haflagah (the same interval again). On each one the couple keeps apart for the onah, and a bedikah is made.
• The minimum days before a hefsek: 5 days in the Ashkenazi custom (Rema), 4 in the Sefardi (Mechaber), counting the day of onset as day one.
• The hefsek taharah: the bedikah made before sunset that shows the flow has stopped.
• The shivah neki'im: the seven clean days after the hefsek, with their bedikos.
• Mikvah night: the night after the seventh clean day.

Paper luach or app?

A paper luach works, and many women still keep one. Its weak spots are arithmetic and time: counting thirty days across a month of 29, finding the same Hebrew date when the month has no 30th, working out an interval in onos, and remembering which veses was uprooted by which period. Mistakes there are easy and invisible.

A taharah calendar app does the counting, but only an app built for these laws can be trusted with it. A regular period tracker counts Gregorian days from the first day of bleeding; it knows nothing of onos, Hebrew dates, the hefsek or the seven clean days. Look for one that works in Hebrew dates and onos, uses the sunset where you are, lets your rav choose between the opinions, and shows you its working.

Keeping it with your rav

Several of these rules are a machlokes: whether to keep day 31 as well as day 30, whether the haflagah is counted from the onset or from the hefsek, whether the minimum is 4 days or 5. A calendar should not decide them for you — it should let you set them the way your rav says, and keep them that way.

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.