Holiday as a singular noun commonly refers to a specific day or event:
It’s a public holiday on Monday, isn’t it, so I suppose the banks’ll be closed?
She seems very tired and upset at the moment. She needs a holiday.
We booked a holiday online yesterday – two weeks in Greece.
We use the plural noun holidays and holiday in similar ways:
We are all going to Croatia for our holidays this year.
We are all going to Croatia for our holiday this year.
We often use on with holiday:
They’re very good friends. We first met them when we were on holidayin Morocco.
Not: …
when we were in holiday… or …when we were at holiday… or …when we were on holidays…