Holiday in the singular form would be used to describe a specific holiday: “What is your favorite holiday, Christmas or Easter?” It would also be singular when describing a vacation: “I’m going on holiday with my family”. It would be in the plural form when describing multiple holidays: “Which holidays do you celebrate?” And plural when describing the holiday season: “What did you do over the holidays?” I hope this helps!

Thank you very much for these answers. It helps me a lot! But in which case would you use the plural of this word?

Here's an example of the plural: Thanksgiving and Independence Day are holidays in the USA.

Also, for what I described, “vacation” is American, “holiday” would be British, which is what you prefer according to your bio.

This is different between America and England. For Americans, any day (sacred or not sacred) where everyone gathers to celebrate something (Christmas, New Years, Independence Day, etc)