Cultures in the non-tropical regions of our planet typically celebrate seasonal holidays such as lights, fire, evergreens, and feasting at the winter solstice; harvest festivals and tributes to the dead in the fall; rituals welcoming spring, e.g., Easter and May Day (as well as advance preparations for the return of spring, such as Carnival and Lent); etc. Heather Rose Jones's Alpennia series takes place in an imaginary country in a version of our Europe. In addition to familiar holidays, the capital city marks the changing of seasons by measuring when the river rises to a certain level. What kinds of holidays might be celebrated on worlds that don't have seasons like ours at all? Come to think of it, why do the Fraggles in the animated series FRAGGLE ROCK have a midwinter festival of bells? They live in a giant cavern complex, where the climate should stay uniform all year round, and they don't have a view of sun, moon, or stars to mark the cycle of the year. (Yeah, I know, because the writers wanted a sort-of Christmas episode, and I loved it, but in-universe the episode lacks logic.)
On a planet where the main division of the year's climate falls between wet and dry, the onset of the rainy season—the time of fertility—might be an occasion for a major holiday. On Marion Zimmer Bradley's Darkover, which has four moons, some festivals coincide with the appearance of all four moons in the sky. Earth cultures mark months and weeks by phases of the moon, and some cultures follow a lunar rather than a solar year. How would the calendar of a world with no moon look? Without weeks in our sense, what method would societies use to set aside days of rest? Or consider a world like the planet in Isaac Asimov's classic story "Nightfall," with several suns. On that world, total darkness occurs only at intervals of over a millennium. With no memory of night and stars except in mythology, people go mad from the unprecedented sight, and civilization collapses at every "nightfall." But suppose darkness happened rarely but not all that rarely, say roughly once a year. The peoples of that world might have holidays to get them through that frightening occurrence, just as ancient cultures on Earth held rituals and celebrations to ensure that the sun would return on the winter solstice. Other kinds of worlds might have holidays centered on the periodic eruption of a geyser, the migration of important species of animals, or the blooming of a special tree. In our own culture we have celebrations such as the Cherry Blossom Festival and (in my home city in Virginia) the spring Azalea Festival. Capistrano honors the return of the swallows.
The STEVEN UNIVERSE animated series (Cartoon Network) takes place in an alternate world similar to ours but with divergences in history and geography caused by the Gem War (an alien invasion) thousands of years in the past. The characters live in Beach City in the state of Delmarva, for instance. According to the show's creator, this world has no Christmas. We've seen that there's no Halloween. (From these clues, we must assume no Christianity and therefore no Easter either.) Apparently they also don't have Thanksgiving. Other than local town celebrations, we don't yet know what holidays they do celebrate. Because they live in a temperate zone with changing seasons, though, we have to expect them to observe some holidays analogous to the ones we know.
Terry Pratchett's HOGFATHER takes place at the season of Hogswatch, Discworld's analog of Christmas. At the winter solstice the Hogfather brings toys to good children in a sleigh pulled by giant boars. People leave sausages instead of cookies for him, in keeping with the origin of Yuletide as a all-out orgy of feasting before the privations of winter. At the climax of the novel, Susan, Death's part-human granddaughter (it's complicated), has to save the original Hogfather, the primal being on whom the myth is based, from permanent annihilation. Death tells Susan that if she had failed, the sun would not have risen. When she asks what would have happened instead, he says, "A mere ball of flaming gas would have illuminated the world." It's also Death who tells us, in the same scene, "Humans need fantasy to be human."
Margaret L. CarterCarter's Crypt