order compared to numbers indicating the day of the week.For Gregorian calendar:
*ignore periods of 400 years
*considering the second letter in the case of a leap year:
**for one century within two multiples of 400, go forward two letters from BA for 2000, hence C, E, G.
**for remaining years, go back one letter every year, two for leap years (this corresponds to writing two letters, no letter is skipped).
**to avoid up to 99 steps within a century, the table below can be used.

Red for