I don't know how the "official" notifications work, but I sometimes see people talking about it a day or two in advance, but not more than that.
Keep in mind that on a free listing day, there are many more items listed than usual. The people who are heavy ebay sellers and have software to automate listings can list hundreds of items at once. That means what you are trying to sell will be competing with many more items, and many of those items will be ending at the same time yours are. It might be smart to vary the auction end times because most people list 10-day auctions.
The bottom line is expect lower prices when you list items on a free listing day.