A well-known mnemonic for calculating days of the week is the Calendar Atbash. An Atbash is a simple cypher where the first letter of the alphabet is replaced by the last, the second by the next to last, and so on. Thus Aleph is replaced by Tof, Beth by Shin and so on; this gives the acronym Atbash.
Applying the Atbash to the first seven days of Pesach, we get
Aleph – Tof – Tisha B’Av
Beth – Shin – Shavuot
Gimel – Resh – Rosh Hashana
Daled – Kuf – Keriat Hatorah, i.e. Simchat Torah, a day devoted to Keriat (“reading of”) the Torah
He – Tzadi – Yom Tzom Kippur, the Day of the Fast of Atonement
Vav – Pe – Purim
Zayin – Ayin – Yom ha-Atzmaut, Israel Independence Day
This is to be read “The first day of Pesach is on the same day of the week as the date beginning Tof, i.e. Tisha b’Av”, etc. (The first line is spoilt if that day is Shabbat so that the fast has to be postponed to Sunday.) Israel Independence Day may also be moved. Note that the Atbash remained incomplete until the creation of the State of Israel meant that this new festival was created.