Thank you for reaching out to us.
The key halachic number for estabishing patterns with
vesatot is three. When a woman has three consecutive flows beginning say, at night on a specific
date of the month, that establishes regularity, a
veset kavua. When it comes to intervals between the onsets of
niddah flow,
three intervals are needed to establish regularity. It takes four periods to create three intervals--the interval from 1 to 2, the interval from 2-3, and the interval from 3-4.
So if on three consecutive occasions your period began in the daytime after an interval of 27 days, then you have established a
veset kavua. If you then have a flow beginning on day 26, you would observe both 26 and 27 from that new day as
veset days for the following cycle. A clean
bedikah on day 26 would remove it from your
calendar, and bring you back to just observing 27.
We hope this is clear. Please let us know if we can be of further assistance.
