Hi Rob
It's a loose "same day" -- essentially it means you got the order together and shipped it while it was still in "Processing," a time period between 24 and 48 hours (depending on when the order was placed) before the buyer can cancel it. It's not particularly sophisticated so it does not grant you extra time for mailing holidays (as these vary between countries.) Sellers who can print a mailing label with a form of tracking from their own business or home have somewhat of an advantage, because you can get it in the mailing system as soon as it's packed (although many USPS locations have self-serve kiosks where you can also get a package mailed during holidays and weekends.)
It's great that our sellers are paying attention to seller performance, though the same day shipping bonus is just a plus, and you needn't be overly concerned if you are "merely" getting orders out within two business days. Sometimes sellers get orders for hundreds of items, and it is pretty miraculous if you can get that all pulled, packed, and out in the mail within 24 hours. The things that will really hurt your seller performance are late shipping and cancellations, so good inventory control and a system that lets you locate your inventory items quickly should be a primary concern.