The busiest day of mailings through the United States Postal Service is expected to be today, and the busiest delivery day is projected to be Wednesday.USPS spokesperson in Louisville David Walton says the holidays are still the busiest time of year for the postal service, but volume has slipped this year.“Compared to last year, we are down a little bit," says Walton. "Locally, our cancellations are down overall about ten to fifteen percent. However, our parcels and priority mail volume is expected to be up about twenty or thirty percent, and that’s due in part to more people shopping online at places like Amazon and Zappos.”Christmas cards also increase mail volume. Walton says the postal service expects to deliver more than three-billion cards this holiday season.