Как правильно, Were you on a holiday? или Was you on a holiday?

Правильно: Were you on a holiday?

Объяснение простое: с местоимением you глагол to be в прошедшем времени имеет форму were, а не was.

Правильные формы в Past Simple:

• I was

• he/she/it was

• you were

• we were

• they were

Поэтому:

• Were you on a holiday? — правильно.

• Was you on a holiday? — неправильно.

Примеры:

• Were you on a holiday last week?

• Were you at home yesterday?

• Were you happy with the trip?

Иногда путаница возникает потому, что с he / she / it действительно используется was:

• Was he on a holiday?

• Was she at work?

Но с you всегда were:

• Were you on a holiday?

Кстати, в современном английском чаще говорят on holiday в британском варианте и on vacation в американском:

• Were you on holiday? — британский английский.

• Were you on vacation? — американский английский.

Фраза on a holiday возможна, но обычно звучит менее естественно, если речь идет просто об отпуске. Чаще говорят именно:
Were you on holiday?