I hope you've had nice holidays or I hope you've had a nice holiday ?

When some one is back to work after 1 week, which of the following applies to that person:

I hope you've had nice holidays.
or
I hope you've had a nice holiday.

I am not a teacher.

I would use the singular 'holiday', but I would probably say it differently.

I hope you had a nice holiday.
I hope you enjoyed your holiday.

I'd use the singular too for a personal holiday. I would use the plural form something like school holidays for all the people at the school.

In AmE, we would say "vacation" unless it was really a "holiday"