Holidays
It's a personal thing, but I never send anything out in December that hasn't been requested. It's more than just busy agents and editors, though many read absolutely nothing after about the 10th. It's also the chaos of the mail system, the fact that things get buried, lost, routed wrong, and sometimes destroyed.
As an editor, I hated receiving anyting in December, and when I worked for an agent, she felt the same. Things that arrived in December seldom moved at all, and usually just got stuck in with a mass of other stuff that all got sorted out sometime in January. December is more than just Christmas shopping and parties, it's also an extremely busy time for agents and editors because this month is when you concentrate on selling books you've already published, and on promoting clients are already work with.
Patience is a virtue, and after being on the inside, I've found it's simply safer, and more productive, to wait until Christmas is gone before mailing anything.