DC has a big 4th of July shindig every year, unsurprisingly. Trump's been begging his aides for a big Salute To Trump parade since he got there, and I guess they finally convinced him that what better day than July 4, a day when DC is already filled with people for something which happens every year. Saves them the trouble of actually having to convince anyone to come, and keeps the boss happy for 3 minutes (look at the crowds, boss, they're here for you!) until the brains worms and soul cancer fight back.