Looking on the internet deeply has found these results:

on summer holidays I is the most popular phrase on the web.

in summer holidays I 138,600 results on the web	on summer holidays I 1275,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I hope that before the summer holiday I will be able to submit a formal motion to make their agreement binding. Like when you change schools, or end the summer holidays. Come and see me in the summer holidays. No matter how hard it is, you can't come home until the summer holidays. The baby's due in the summer holidays and I'll go back to school and you can take care of the baby. It was on my summer holidays. Come and see me in Tokyo in your summer holidays. I can't wait for the summer holidays.	I went to Bali on summer holiday. No, he's been away on summer holiday. I can't wait for the summer holidays. I hope that before the summer holiday I will be able to submit a formal motion to make their agreement binding. Like when you change schools, or end the summer holidays. No matter how hard it is, you can't come home until the summer holidays. It was on my summer holidays. Come and see me in Tokyo in your summer holidays.

in summer holidays I

138,600 results on the web

on summer holidays I

1275,000 results on the web

More popular!

I hope that before the summer holiday I will be able to submit a formal motion to make their agreement binding.
Like when you change schools, or end the summer holidays.
Come and see me in the summer holidays.
No matter how hard it is, you can't come home until the summer holidays.
The baby's due in the summer holidays and I'll go back to school and you can take care of the baby.
It was on my summer holidays.
Come and see me in Tokyo in your summer holidays.
I can't wait for the summer holidays.

I went to Bali on summer holiday.
No, he's been away on summer holiday.
I can't wait for the summer holidays.
I hope that before the summer holiday I will be able to submit a formal motion to make their agreement binding.
Like when you change schools, or end the summer holidays.
No matter how hard it is, you can't come home until the summer holidays.
It was on my summer holidays.
Come and see me in Tokyo in your summer holidays.