I should probably preface this by explaining a bias of mine. I don’t think it overly affects my enjoyment of the episode, but I can’t say it has no effect. I have always hated halloween and I’ve never understood in even the slightest degree the impulse in others to willingly seek out being scared or frightened, from which somehow, by some arcane alchemy I can’t wrap my head around, they transmogrify into amusement and fun. I have a similar dearth of understanding regarding thrill-seeking, such as taking rides on rollercoasters, which I also cannot abide, and so I suspect (but still cannot fully comprehend) that it has something to do with people deriving pleasure from adrenaline. This probably means the ponies have a similar adrenaline response, since several characters in the episode express that they sometimes find it fun to be scared. Our episode opens with Spike, in his rather redundant dragon costume, telling Twilight they’ll be late for Nightmare Night and not recognising her Starswirl costume. As most people no doubt note here, not only does Spike or the rest of Ponyville not recognise this figure, but furthermore, Twilight mentions him as having a shelf at the Canterlot Library of Magic named after him, being the father of the amniomorphic spell, and being the most important conjurer of the pre-classical era… Not, notably, the mentor of Celestia and Luna, and also not even as the most important conjurer ever, much less most powerful unicorn ever, but merely the most important of his era. She mentions that he created over 200 spells, and again this mention is notably not “unicorn that created more spells than any other in history”. Even his name, Starswirl the Bearded, implies that there’s potentially at least as-well-known figures called Starswirl in history. At the very least he seems somewhat less significant than he might later seem to be.
The cold open presents Twilight frustrated at not having her costume recognised, and it's also from an older, more obscure time in Equestria's history. The cold open concludes on Spike jokingly calling her 'grandpa'. It's more subtle than a lot of the cold opens we've had, but it ties into the themes of Luna being behind the times and not recognised for who she actually is, or even was. Nightmare Night has made her into a very different figure in the minds of contemporary Equestria. Some of the houses as Spike and Twi walk through town look like they've had their roofs dressed up, like hats with bells and such. When it comes time for Rainbow Dash and Applejack to comment on Twilight's costume, Applejack thinks she's a country-music singer due to the long beard. I don't know much about really any music generally, but I was wondering if that was something like a ZZ Top reference? I've had people call me ZZ Top because of my long beard, so all I really know is they're a band of some sort famous for long beards.