I was looking up https://en.wikipedia.org/wiki/Pope_Sylvester_I recently (no points for guesses as to why), and was surprised to learn, en passant, that catholics simplified in the 1960s and now only have 4 different classes of feast days: https://en.wikipedia.org/wiki/Calendar_of_saints#Ranking_of_... ; I imagine whatever came before was slowly accreted over the centuries and, however simple it started, had eventually relied on people having lots of time to learn all its subtle corner cases...
There have been a few rounds in the Church's history of gradual accumulation of feasts followed by a simplification.
A little while back I wrote some code to calculate a liturgical calendar based on pre-60s rubrics. [1] It was a fairly complicated project. There have been a few edge cases that have come up over the years. (E.g., if Christmas Eve is on a Sunday, does it take precedence over the 4th Sunday of Advent? If the Feast of St. John falls on the Feast of the Sacred Heart, which takes precedence?)
A little while back I wrote some code to calculate a liturgical calendar based on pre-60s rubrics. [1] It was a fairly complicated project. There have been a few edge cases that have come up over the years. (E.g., if Christmas Eve is on a Sunday, does it take precedence over the 4th Sunday of Advent? If the Feast of St. John falls on the Feast of the Sacred Heart, which takes precedence?)
[1]: https://github.com/joe-antognini/tridentine_calendar