Monday was actually the busiest day of the season for the Post Office, and the last day that people could send packages and cards first class. The Post office was expecting almost 40 percent more mail for the day than a typical Monday,.
If you missed the Monday rush, don't worry. If you have the money, you can still get those gifts there on time.
Tuesday, December 21st is the last day to send out on Priority Mail. If you still can't make it then, your best bet is to send out on Express Delivery, with the safe deadline being December 22nd.