As I found out upon awakening, the weather did prevent a launch of Endeavour on Sunday. Clouds. The next launch opportunity is at 4:14 AM Eastern Standard Time (0914 UTC) early Monday morning.
Decision time on tanking is 6:15 PM Eastern, with tanking to begin half an hour later if approved. The cloud cover remains iffy for a launch.
If it sounds as if there are a lot of weather criteria for launch, it's because there are. The list is huge. It covers not only the launch site, but the Eastern Test Range, the Booster Recovery Zones, the Shuttle Landing Facility (RTLS abort), and the TAL (Trans Atlantic Landing abort) sites. The list is here.
It takes a fair amount of luck before they're ever able to go at all.