Metro is celebrating National Dump the Pump Day, which is this Thursday, June 19, by giving away one $100 SmartTrip card to the lucky person who can guess how many people will use the Metrorail system that day. If you’ve got a guess, you can submit your number on Metro’s web site. Entries must be submitted before midnight on June 19. They’ve got a a handy chart posted if you’re completely in the dark…