Is the 4:00 and 6:00 Christmas eve mass super crowded? How early do we need to arrive?

no less than 30 min before the mass.

Christmas Eve masses are typically crowded. Plan to arrive no less than 30 min before the mass.

If you want to sit, during the Mass, I recommend at least 1 hour prior to the 4 pm service. As for the 6 pm service at least 30 minutes. There will be music while you wait for Mass to start. Hope this helps.