How to Determine Pascha
Pascha falls on the first Sunday after the first full moon after the vernal equinox. The vernal equinox is March 21, so Pascha can never fall earlier than March 22. The latest that Pascha can fall is April 25. These dates are 34 days apart.
Once these two dates are determined, the earliest and latest possible dates for all the other feasts that depend on Pascha can be set. This cycle of the moveable feasts based on Pascha is called the Paschalion.
Working backwards from Pascha, we find that the first feast of the Triodion, the Sunday of the Publican and the Pharisee, can fall as early as January 11 and as late as February 14, with all the other feasts of the Triodion following.
Working forward from Pascha, we can determine all the feasts of the Pentecostarion: Thomas Sunday can fall as early as March 29 and as late as May 2, and so forth.
The latest date on which each of the feasts of the Triodion and Pentecostarion can fall is 34 days after the earliest. The following page shows that difference only for the date of Pascha, by which everything else is set.