Hebrew or Gregorian date to look up.
true for the Israel schedule (no 8th day Pesach,
no 2nd day Shavuot); false for the Diaspora schedule.
An array of one or two chapter numbers (1-6), or null
if dt is not a Shabbat, is before Pesach, is after Rosh
Hashana, or coincides with a holiday on which Pirkei Avot is
skipped (8th day Pesach / 2nd day Shavuot in the Diaspora; Erev
Tish'a B'Av or Tish'a B'Av).
Returns the Pirkei Avot chapter(s) studied on the given Shabbat, or
nullif the date is not a Pirkei Avot Shabbat."From at least the time of Saadia Gaon (10th century), it has been customary to study one chapter a week on each Shabbat between Passover and Shavuot; today, the tractate is generally studied on each Shabbat of the summer, from Passover to Rosh Hashanah, the entire cycle repeating a few times with doubling of chapters at the end if there are not a perfect multiple of six weeks" (https://en.wikipedia.org/wiki/Pirkei_Avot#Study_of_the_work).
Unlike most other calendars in this package, this one is purely computed from the Hebrew calendar — there is no fixed start date, just a recurring seasonal window. It never throws on a pre-cycle-start date; it returns
nullfor any date outside the window described below.