In the summer I will add a few days as a holiday and call it Summer Break. I will do the same in Winter, and call it Winter Break.
If you are looking for some ideas, I found quite a bit at these links.
Custom holidays ideas for your Sims 4 Calendar
https://dreaminginsimlish.wixsite.com/thesims/post/custom-holidays-ideas-your-sims-4-calendar-the-sims-4-seasons-required
Sims Community - I filled the entire Calendar with Holidays in The Sims 4 Seasons
https://simscommunity.info/2018/07/25/i-filled-the-entire-calendar-with-holidays-in-the-sims-4-seasons/
SimVip - Six Holidays You Should Make
https://simsvip.com/2018/06/22/six-holidays-you-should-make-with-the-sims-4-seasons/
https://www.youtube.com/watch?v=NbisHfYUFu8
