Why does USPS allow us to print labels for days they are closed?

I print all my shipping labels with USPS Click-N-Ship and for the most part Paypal shipping. A few weeks back I was packing and shipping on a Sunday night for about 15 orders. One of the orders I forgot to change the date for the label from Sunday June 7th to Monday June 8th. I figured NBD the people at the post office are usually OK with everything I drop off because they know my face. Well Monday morning the angry lady is there and looks at each box and refuses to take the box with the Sunday date! Here is my question: If the post office won't accept boxes with labels that have dates indicating a prior date to dropoff, WTH do they allow us to print these dates? USPS do us a favor and have your programmers remove Sundays and holidays from the drop down boxes?