Tough to say... last year the party sold out only on Oct 31. This year, the party sold out on Sept 29 (if I'm remembering correctly), but many speculate that was because of it being a Friday, with the free dining promotion, and Epcot F&WF.
So long as the party doesn't sell out, you can get tickets the day of. A better idea, if you're sure you want to go, is to call ahead and buy the tickets now, and have them held at Will Call at guest relations at the MK. That way, you can know for certain the tickets are paid for and ready for your use on Friday. Also, you will get a small discount buying in advance, than buying in person at the party.