I get the Christmas/special popups every time I boot up my system as well, and continue to pop up throughout the day. They've been around for a couple months now, but are getting increasingly repetitive to deal with. I update the Avast program itself on a frequent basis, and of yesterday, the popups have stayed the same... there doesn't appear to be any way to turn them off. The free version is already stripped of enough features, it doesn't need 'dumbed down' by these ads.
Running Windows 7 64bit.