I printed postage tonight, and accidentally left the default date as today (11-11).  The postage came out as 1st Class mail.  I know that the post office is closed tomorrow, so that it won't really mail out until 11-13.  Is there something I can do about this since I've already paid for the postage?

Take it to the PO and ask them to attach a zero postage stamp to correct the date.

Or if you have a blue box available, drop it in there before the first pickup tomorrow. Mail in those boxes, they usually allow outdated postage IF it would have gone out of date if dropped off and then waiting to be picked up.

For instance, if you'd dropped it in the box yesterday (11/11), the date would have been right. But by the time outgoing from that box is picked up tomorrow, the postage would have gone out of date through no fault of yours - you mailed it on the right date.

The PO knows that and generally cuts a little slack for dated mail dropped off in boxes.

If you can print a postage meter label with 00 on it from your own postage meter machine, you can put it right next to the stale mail postage label and the new 00 will update your package to the the new date.  This saves you from having to go to the PO and maybe getting a lecture from a clerk.

how can we print 000 on printed postage?  I dont think we can

You can't with the PBS postage but some people have access through work to an actual postage meter.  You have 48hrs to mail the book.  If you put a book in the blue box after the last pickup it should still be allright even if the next day is a holiday. Once it's in the blue box it's in the hands of the USPO and considered mailed.  I do this all the time and have never had one come back.  Otherwise you will have to wait, take it to the PO and have them put a new date on the package.