AtBash
A”T ( א=ת) = The first day Pesach falls out on A will be when Tisha B’Av falls out
B”Sh ( ב=ש) = The second day of Pesach will be the same day that Shavuot falls out
G”R (ג=ר) = The third day of Pesach will be the day of the week Rosh Hashana falls out
D”Q ( ד=ק ) Day 4 – Simchas Torah (Quf is Qedushat Torah)
H”Tz (ה=צ) Day 5 – Tzom (Yom Kippur)
V”P (ו=פ) = Previous Purim
This is what the Shulchan Aruch wrote.
Nice addition:
Z=’A (ז=ע) – Yom Ha’atzmaut