2Likutey Moharan 74 – 75 – 76 – Preparing for Geulah Begins in Adar – Luach Atbash. Speaker: Rabbi Nasan Maimon (Lesson 460, recorded on 1987-02-01).
To know what day of the week each holiday falls each year: the calendar “Luach AtBaSh”. AtBaSh is system of gematria in which the first letter of the Hebrew alphabet is replaced by the last, the second by the next to last, and so on. Thus Aleph is replaced by Sof, Beis is replaced by Shin, creating the acronym AtBaSh.
Applied to the first six days of Pesach:
Aleph – Tof – The first day of Pesach will be the same day of the week as Tisha B’Av
Beth – Shin – Shavuos
Gimel – Resh – Rosh Hashanah
Daled – Kuf – Krias Hatorah, i.e. Simchas Torah
Heh – Tzadi – Yom Tzom Kippur, the Fast of Atonement
Vav – Peh – Purim