I believe jax was talking about the days of the week rosh hashana and pesach can not begin on. I think rosh hashana cannot begin on:
A-first day of the week-Sunday
D-fourth day of the week-Wednesday
U-sixth day of the week-Friday
Pesach cant begin on:
B-second day of the week-Monday
D-fourth day of the week-Wednesday
U-sixth day of the week-Friday
(when you spell out the words adu and badu in alef-beis, it works)