December 24, also known as Christmas Eve, is not a federal holiday, so mail is delivered normally. The United States Postal Service treats this day as a typical mail day.
The Postal Service keeps a list of postal holidays on its website where people can see what days the service won't deliver mail. If a particular day isn't on that list, then that means it is business as usual.
The United Parcel Service recognizes Christmas Eve as a holiday, but it's only a recognition. As it is not an observance; it is a normal day of operation for the private delivery service. FedEx makes deliveries as well.