I recall seeing Legendary had a sale in December (I think it was 20% off) but I wasn't planning on purchasing upholstery at that time. I would like to purchase new upholstery in the coming months and wanted to ask if anyone knows when Legendary typically would have their next sale or do they only do it at Christmastime?

You're probably going to have to contact them directly, to see what
sales specials ( if any) they may have in the near future.

do they still have a half year wait time?

The last order I placed with them was 1/24 so just over a year ago and lead time was 285 days. Someone else may have a more recent experience but I have placed an order the last three years in a row during their holiday sale and the lead time went down just slightly each year

I can't recall if it was Memorial Day or July 4 or maybe both but in years past yes there were sales like that as well. Also they have offered a discount at big shows, I ordered my stuff at Carlisle 2023 and they were like 9 months out then but I received everything a little bit sooner.