Last week we has the Spring and Summer festivals. Now onto the other two seasons.
Like the spring festivals, we have two in the fall: Nightmare Night and the Running of the Leaves. The Running of the Leaves feels a lot like the exact opposite of Winter Wrap Up. In this case the ponies are removing the last of Fall to make way for Winter. It’s a time of games and other events, but it doesn’t seem to have the same emotional significance as it's Spring counterpart. It's long past harvest time for most crops which occurs anywhere from the beginning to the middle of fall depending on the crop, so it's not a Harvest Festival. While all we see is the single sporting event the festival is currently taking it's name from, there are likely other festivities wrapped around it given the nature of the event itself. So it’s not the equivalent of Thanksgiving as celebrated in America or Canada (different dates, but the basically the same harvest festival. Or in America's case the 'Oh heck, we missed the harvest' festival.), but a much lighter version based on sport. So to translate this into terms more familiar to Americans, this would be more like the first day of football season turned into an official holiday of its own. Complete with elaborate tailgating feasts, award ceremonies and the like. Actually, now that I think on it, it wouldn't surprise me to find that the Equestrian Games grew out of the Running of the Leaves in much the same way the Marathon was the premier athletic event in the first modern Olympics. The Games don't count as a festival for this purpose, as likely it is following the real-life Olympics in occurring only once every four years.
Nightmare Night on the other hand, like Halloween in real life, is a very strong contender for a true Harvest Festival. The sacrifice of food in return for safety, the preponderance of death imagery to point out the ‘death of the year’ (signaled by all the plants 'dying' by going into hibernation mode), so on and so forth. The costuming of Halloween is traditionally to pretend to be one of a variety of evil sprits, either to hide from them when they are the most active, or to capitalize on public fear and extort gifts, eventually in a fun and mocking way as the real fears turn to pranks. In Nightmare Night, the ponies are nominally hiding from a singular spirit, Nightmare Moon, who has taken on a Jack of the Lantern role as I stated here. However, the only way to hide by wearing costumes would be if there were other spirits to hide among. So this indicates that the myths likely have Nightmare Moon having a collection of servants and hangers-on. This might contradict the idea that Luna took on the persona of Nightmare Moon out of loneliness, but it also might mean that these followers of Nightmare Moon came *after* the transformation. Leading to the reinforcement that Luna had to remain as Nightmare Moon to keep the attention she was now receiving. Creatures attracted to the overt use of power that Nightmare Moon demonstrated, perhaps. Creatures now considered monsters; minotaurs, manticores, changelings, bat ponies...
Now, you would think that the ponies would put Nightmare Night and it’s equivalents on the longest night of the year, but it apparently isn’t as that would be a winter festival, not a fall one. Unless Equestria has very asymmetric seasons (which is technically possible given the ponies ability to manipulate the weather) this festival is placed on the Summer side of the Fall Equinox when nearly all crop harvest will be complete, mid-way between the longest day and longest night. The other choice would be to put the festival on the day Luna succumbed to her jealousy and became Nightmare Moon. But the flashbacks in the Season 4 opening episodes seemed to indicate that the time elapsing between the creation of Nightmare Moon and the exile to the Moon was hours at best. So that would be the same day as the Summer Sun Celebration. That curtails a lot of possibilities for worldbuilding, so it may be necessary to do some creative editing.
One possible solution to this is that the flashback was the Cliff Notes version of the events, and the battle between Nightmare Moon and Celestia actually took place over nine months or more with a great deal of retreating, advancing, testing, with random day/night cycles as one force temporary overpowers the other, and so on. Events that are unmistakable and have to be explained somehow to cause the creation of mythologies. Otherwise this is a momentary eclipse and the loss of a princess that might be a footnote in a historical account somewhere. Not something ponies would build two major festivals around.
Nightmare Night is very much Luna’s day. As much as I would like to say the next holiday is hers, as the longest night, her association with Nightmare Moon just won’t let that be.
Winter festivals. While the modern attitude towards winter festivals is of joy and gift giving, the older form was more pleading for survival and clinging to the support of your community during grim, dark times. For example, the lighting of each home’s central fire from the candles taken from the communal tree (an old Celtic ritual that survives in the decorated and lit Christmas tree and the Yule log) reinforced the idea that individuals needed what only the entire community could provide to survive the winter. Another example of the grimness of the festival being the paradoxyl winter feast; Making a party out of the last of the foods that could not be preserved for one reason or another, and so had to be consumed before they went bad.
This dark, yet celebratory situation is very present in the Hearth’s Warming Eve celebration. The celebration of the founding of Equestria and the defeat of the Windigoes, but more importantly the survival of the various ponies and their newfound friendship and community. This is very definitely the Equestrian dark-days festival, and the equivalent to original festivals that occupy the time now taken by the various big winter holidays like Christmas. It’s very likely that the Hearth’s Warming Eve story is mostly fictional but based on a kernel of something real, in the same way Santa Claus is based on a variety of actual people blended with liberal amounts of far older mythology. It would be easy to dismiss the Hearth’s Warming Eve story as completely fictional, but Twilight references what she believes is an actual historical figure (Starswirl the Bearded), and connects him to one of the central characters of the story (Clover the Clever). Twilight is not the kind of pony to let that kind of thing slide. Her OCD nature is central to her character and her fandom-like obsession with Starswirl would lead her to clearly demark what she believes to be fiction from what she believes to be fact. If the story was completely fictional, she would be telling everyone that every five minutes during the episode.
In any case, winter festivals like Christmas and Hearth’s Warming Eve, are primarily about reinforcing social ties and bringing together communities. Being with *friends* and family.
For this reason, when (if) she has reached the same status/power as Celestia and Luna, this will be Twilight’s day, as the Princess of Friendship. Nominally, I would prefer to switch Luna and Twilight here, simply for symmetry, but life is rarely so nicely symmetric, and that does give a bit more 'realism' to this setting. It's not perfect, it's wonky and unbalanced, which is how real life is.
Now you probably noticed that there were only seven festivals here, and winter only got one. That's kinda an accident in symmetry as the two festivals on either cusp of winter (Winter Wrap Up, and the Running of the Leaves) were very distinctly Spring and Fall festivals, not winter ones. Otherwise I would have counted Winter Wrap Up as a Winter festival, and the Grand Galloping Gala into the Spring to keep the everything symmetric. Still, we're missing some kind of fete on the cusp of Summer and Fall. This isn't surprising as there are likely a number of holidays and celebrations that we haven't seen yet. Especially ones from the other cultures like the zebras, the donkeys, the Saddle Arabians, the gryphons, etc. So likely we'll see something in the coming season as the writers do like showing festival-like celebrations but seem to be avoiding revisiting ones we've seen before. Additional holidays will possibly be more secular in nature, celebrating historical events that we're unaware of at the moment. Such as the Equestrian equivalent to Guy Fawkes day, or more frantically party-like celebrations like Mardi Gras or Holi (also spring festivals, but with a less work more play energy than Winter Wrap Up. These might make good choices for cusp of Spring and Summer festivals, especially if we get more information that moves the GGG over to the Summer/Fall slot).
So, very likely this topic will get revisited at some point soon.