There were a rare number of instances (after the hurricane) where tickets to a hitherto sold out party were available for a few hours the day of. Where you could purchase them seemed to depend on the skills and knowledge of the person at the window. After the hurricane this year, another poster was able to buy them at Epcot guest relations. However, I was told no at DHS guest relations, and that I had to go to the Magic Kingdom to complete the purchase. Fortunately, I was able to avoid the extra trip and do it on my phone. So…mileage will vary.
That being said, barring a weather event with a large amount of cancellations, I doubt they’d suddenly have extra inventory to sell the day of. However, it can’t hurt to try at the window that day if you really want to go. The worst they’ll tell you is no.