Jack Applin writes:

“When Labor Day falls at the end of the first week”? Where does Frazz take place? In the United States and Canada, Labor Day is the first Monday in September, which isn’t at the end of a week.
https://en.wikipedia.org/wiki/Labor_Day
https://en.wikipedia.org/wiki/Labour_Day_(Canada)

I’m guessing that what Mallett means is “When Labor Day is on the 6th or 7th of September”, i.e., near the end of the first week of September. But I still don’t get the joke even if that’s the intent.