My store lets the seasonals go the 1st week of January - that is the week with the last shifts for each seasonal not being kept.
After Christmas, the seasonals are told whether they are being kept or not. They do not tell seasonals before Christmas so they don't have a bunch of team members calling out and NCNSing right before Christmas when the store needs everyone.
After the seasonals are told, there are always a few that just stop showing up. Never made sense to not work the last couple shifts so you can be eligible to come back in the future if you wanted. The seasonals that stop showing up are labeled as not rehireable.
The seasonals are used to restock and straighten up the store for a week or two.