Celebrations to have in your fictional society
Across humanity there are some things most known cultures have celebrations for and can be used well as an easy way to expand on the lore of your world without info dumping. here are some of them:
Independence / Unity / National / Freedom Day: the day used to commemorate when a nation, kingdom, or faction came together as one. This may be the day a treaty was signed, the day the former regime was overthrown, the day a new ruler was sworn in...
Ruler's Birthday: if there is a single or a few rulers, their birthdays might be celebrated with large festivities
End & Beginning of a year: regardless which calendar is used, most societies had some way to measure the length of one cycle (lunar cycle, every ___ amount of days, the beginning of a particular season, maybe something more abstract like the first day the flowers bloom or the day of the first snowfall...) and celebrate accordingly
Celebrating a Season: celebrating summer with festivities outside, spring with a flower market, autumn with a harvest festival, winter with a bonfire...
Day of Mourning/Remembrance: either in general or a day for remembering the dead from a certain event, tragedy, war...
Religious Festivals: depending on your type of belief system there could be many celebrations or only one or two
Historic Memorial Days: the day a war ended, the day of a massacre, the day of a revolution, the day of any grand and influential type of event that is remembered years later
Labour Day: a day to celebrate those who work hard to actually uphold the system and makes up the vast majority of the population, often accompanied by protest and conversations/demands between leader(s)/speakers of the people and the head of state