WARNING: The following contains spoilers for Episode 11 of The Slime Diaries, "Where Is Santa Claus?" now streaming on Crunchyroll.
The spin-off of That Time I Got Reincarnated As A Slime focuses on day-to-day life, which, most of the time, is fairly boring. But The Slime Diaries is playing really heavy into festivals, having featured three already. Rimuru has hosted the Summer Festival, the Bon Festival, and the Christmas Festival, much to the admiration of his citizens in Tempest. It still seems like a lot, considering the anime has only aired 11 episodes thus far. Rimuru must really like these events -- the question is why.
Rimuru, once known as Satoru Mikami on Earth, was transported to this fantasy world after being killed by an unknown assailant and reincarnated as a slime. With the original anime is in its second season, Rimuru has been in Tempest for a while now, surrounded by monsters and creatures unheard of in his previous life.
Therefore, it's understandable that Rimuru is missing some things from his old world, which becomes apparent in The Slime Diaries. First, he hosts the Summer Festival and wishes they could also have takoyaki and fireworks, which are products of Earth. Yet somehow, his citizens pull through and make that nostalgia come back for him, trying to replicate the things Rimuru misses most as best they can.
But the festivals aren't just about what Rimuru wants. He is trying to instill a sense of community in Tempest, a town made up of many different types of monsters all living together. So he hosted the Bon Festival, an event for taking stock of one's family and history and connecting with the community. While Rimuru may have gotten something out of it, the festival's intention was to bring together all of the citizens, and it succeeded.
These events also serve a purpose to the plot. We get to see different sides of characters, such as Geld or Milim, that we wouldn't otherwise see. They are shown in new lights due to the festivals' nature, a time when people come together despite their differences. The festivals are also never short on good food and drink, which every citizen of Tempest loves.
Altogether it may seem like The Slime Diaries is full of festivals, but they serve an important purpose for the anime. These events continuously bring together diverse groups of monsters and instill a newfound sense of community with one another. It helps create bonds through fun and camaraderie that otherwise would have gone untended. While Rimuru may have some selfish intentions behind these festivals, they bring every character a sense of purpose, gratitude, and fun.