A WebmasterWorld thread asks whether users are noticing any sort of "AdSense slump" or a "a little dip in U.S traffic & revenues."
Indeed, people are, with earning drops between 20% and 95%, and it's not atypical behavior for the three-day weekend.
You won't find a lot of folks in the U.S. sitting in front of their computers during a 3-day weekend.
What else can possibly contribute to the lower earnings? There's better weather, college graduation (and thus slower Internet connections), the Memorial Day holiday, and summer vacation.
Forum discussion continues at WebmasterWorld.