When you buy your postage on ebay or paypal, there is a box that asks what day you are planning on mailing, so you shouldn't make that mistake. I wonder if they will let them slip through in the first few days.
I believe there's a grace period.
Well, this is funny. After having this this discussion here, one of my items just sold and I went to print the shipping label. It's under 1 lb. so it's going First Class. But I MADE THE MISTAKE of not changing the shipping date to Monday!

Had to void the label and reprint. When I changed the shipping date to Monday, the rate changed to the new zone-dependent amount.