We use the singular holiday tor a short period of, say, one or two days.
- We’ve got a holiday next Tuesday.
We get five days’ Christmas holiday this year.
We often use holidays for the ‘big holiday’ of the year.
Where are you going for your summer holiday(s)?
We always use the singular in the expression on holiday. (Note the preposition.)
- I met her on holiday in Norway. (NOT . . . in holidays . . .) Americans use the word vacation for a long holiday.