Why is Christmas/Yule/Saturnalia a few days later? Why is the New Year a week and half later? Why is the 12th month called the 10th in Latin? Why are these dates all out of line with each other and scattered around?
It’s all because “the Romans had no fucking idea how to run a calendar”.
You think you’ve heard all the ad hoc arbitrariness about how the Romans managed their calendar as you’re led step by step through it all, and then you learn about Mercedonius and you just want to throw everything on the floor and walk away.