The first one on the show... they forgot about it, which upset Martin, a little...
The second one was during the black out and Martin said that he preferred the last one, IE when they forgot about it.
The third one, was when Martin introduced Sherry to his sons and Frasier had to remind Niles that it was coming up
The fourth one, we hear about a gift competition that was going on (presumably for a long time) and getting out of hand, and ended in a pact about a spending limit the last time.
I have only one question: What?????