South Beach is 'bout to be mobbed. It's almost Memorial Day weekend in the 305 (otherwise known as Urban Beach Week), the annual four-day span during which the beach (as well as mainland Miami) gets swarmed by famous rappers, celebrity strippers, and 250,000 party people from all over the Dirty South. Now, some Dade County residents choose to flee for the holiday. And this year, certain SoBe clubs are even choosing to close till the Urban throng leaves town.