WASHINGTON — The very idea that the National Mall could be packed any tighter seems unfathomable.It was shoulder-to-shoulder from the Lincoln Memorial to the Washington Monument as more than 500,000 people jammed to Bruce Springsteen, John Mellencamp, John Legend, U2, Stevie Wonder, Garth Brooks, Usher, Beyonce and others and listened to speeches from the likes of Denzel Washington, Tom Hanks, Kal Penn (Yes, Kumar from “Harold and Kumar go to White Castle”), Martin Luther King III, Tiger Woods and Samuel L. Jackson.This was ObamaStock — also known as “We are One: The Obama Inaugural Celebration at the Lincoln Memorial.” It officially kicked off the inauguration festivities leading up to Tuesday’s swearing in of Barack Obama as the 44th President of the United States.On Sunday, nobody seemed to mind getting a little cozy with the strangers packed in around them. Nobody seemed to mind the temperatures that lingered in the upper 20s or lower 30s for most of the day. Instead, they wanted to celebrate.Or they just wanted to listen to a few music legends perform for free.Whatever their reasons, they got there by foot, plane, bus and train and packed the National Mall seemingly to the breaking point.Only it’s going to get more crowded.The steady flow of airplanes into Reagan National Airport – one flew overhead every two minutes or so – added weight to the realization that Tuesday’s inauguration festivities will be much more congested. As busy as it was Sunday, people still have today to arrive before the real party begins.