Ah, the American small business. Half of it is a starry-eyed dream built on rainbows and elbow grease; the other half is a desperate shell game trying to stay one step ahead of the bank. Because of the often agonizingly razor-thin margins for small businesses like catering, the deposit's there to protect them if you decide to pull out at the last minute, not to guard you against them going bankrupt. So, if your caterer is about to go under but doesn't know it (or even if he does but is clinging to the last shreds of hope), you might be losing both the deposit and the food for your big event.
Americans mostly celebrate Memorial Day as the start of summer. HowStuffWorks explains the annual holiday's history that's worthy of acknowledgment.