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 …