Ubisoft says that everyone who bought Assassin's Creed 3 should download the "day-one patch" because it "optimizes player experience and is highly recommended." Not exactly an inspiring message for a product that you just bought. The day-one patch fixes bugs and other glitches that shipped with the game, apparently. Ubisoft has a list of fixes here in the patch notes, but you probably won't want to read it because it is chock-full of spoilers. The long and short of it is that the day-one patch fixes a number of game play glitches that might break the game for you like problems with NPC placement, problems with exiting areas or locations in the game, and other mission specific problems.
On a related note, Hurricane Sandy shut down a number of midnight launch events on the east coast for the game at retailer GameStop as the storm slammed into the United States and saw New York City go dark yesterday. On Monday afternoon GameStop posted a note on its Facebook page telling gamers that it had cancelled all of its midnight launch parties for Assassin's Creed 3 at all of its northeastern US locations that were in the storm's path and urged them to stay safe and out of harm's way.