I apologize in advance, because this will be pretty long. I already have a vague idea of some customs, so I need to include both the sociopolitical aspects of this as well as those customs.
The social hierarchy consists of five social classes: Solars (the reigning king and queen), Stellars (royal children), Lunars (the aristocracy), Perihelions (employees of the crown, basically any type of government worker), and Terreans (the working class, typically either employed by Lunars or Perihelions, and make up the majority of the population.) Their society is human, and their kingdom (Corona) occupies the grasslands of a continent, which lends itself to their majority agricultural economy. There are no other human societies in this world, so Corona isn't competing with any other economies or political interest.
The Coroneans worship the suns (their solar system is binary, with their planet orbiting two stars), and their aforementioned social hierarchy is named after celestial bodies.
The Coronean calendar has five seasons, with the traditional four as well as a fifth "twilight interval" called Nox. In this season, Corona is blocked from the sun and receives no light. As this is the case, many plants have adapted to darkness and are bio-luminescent, as well as some animals. The people of Corona have adapted to have markings on their cheeks that glow gold in total darkness. They believe that their markings connect them intrinsically to the suns and their magic, power, and energy, and that the king and queen are direct descendants of the suns. Because of this belief, the monarchy is stable - a form of the Mandate of Heaven. At the beginning of Nox, the Coroneans have a massive holiday, in which they thank the suns for their light and humble themselves to prepare for three months of darkness. During this festival, if the eldest Stellar has come of age, the king and queen will announce their retirement and begin preparations for the Stellar's ascent. (The actual coronation ceremony is known as the Ring of Light, as the crown is reminiscent of the sun's corona).
Most of the holidays I've thought up revolve around the calendar, due to their worship of the suns. Any ideas for some holidays that might affect the lower classes specifically? I toyed around with making the end of major crop harvests holidays but couldn't figure out a way to make them line up well on the calendar. Thank you in advance and sorry again for the long read!