In what seems to be the most celebrity-birthday-packed day we can ever remember, three stars in the forefront of pop culture are going to be blowing out candles and celebrating tonight. On December 18 in varying years, megastars Brad Pitt, Christina Aguilera and Katie Holmes were all born ... as were two guys who have done a few minor things here and there -- Stephen Spielberg and Keith Richards. What's in the air on December 18 that produces such star power? Since we're not doctors, we'll chalk it up to coincidence, but either way, happy birthday to everyone born on this fine day!