Milestone Celebration: "Treehouse of Horror XXVII" is the 600th overall episode of The Simpsons, which tied with Gunsmoke as the longest running American scripted primetime series (recognized by one of the joke credits, which reads "We're coming for you, Gunsmoke"), and acknowledges this at the end of the opening sequence, where Frank Grimes warns that you'd be forced to watch a marathon of all the episodes in Hell.note Likely a cheeky reference to FXX's 600-episode Simpsons marathon later that year.
"Treehouse of Horror XXX" is the 666th episode of the show overall, though it did have a joke about being the 667th in the event FOX changed the schedule, which didn't end up happening.
Out of Holiday Episode: Fox has been generally unable to schedule these episodes within the week before Halloween since buying the exclusive rights to the World Series in 2000 (except in 2009), leading to most of the latter-day specials airing early in November instead. Several of the later specials lampshade this (the opening sequence in XIV has Kang and Kodos express bafflement that they're showing a Halloween episode in November, when they've already got their Christmas decorations up, and it becomes the basis of the entire intro in XVI) although starting in 2011, this problem has since been fixed (although it has occasionally resulted in the episode airing at the very beginning of October instead).
Short Run in Peru: "Treehouse of Horror XXXI" first aired in Canada's Citytv on October 18, 2020. While originally, this wouldn't be that major, since the episode was scheduled to air the same day on Fox, it was later delayed to November 1st, due to Fox's coverage of Game 7 of the 2020 NLCS and the World Series.
The first ten Treehouses were made at the beginning of the production season. However, they eventually became such a drain on the writing team that they were moved to near the end of the season and held over until the next broadcast season so they didn't leave the writers feeling too burnt out so early on. Creative burnout is also the reason why they switched from having a different writer for each segment to having one writer for the entire episode after XIII; having three different writers handle each act resulted in a massively overlong first draft that took ages to cut down.