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on summer holiday is the most popular phrase on the web.

in summer holiday 11,520,000 results on the web	on summer holiday 18,600,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Do come and see me in 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. Come and see me in the summer holidays. In doing so He proposed bringing the elections forward from June to May (in order to avoid summer holidays in Northern states), and to truncate the time for polling itself to two days maximum. Mrs Theorin referred to what we in Sweden call summer holiday circumcision, which is when girls are taken abroad to be circumcised. Actually I'm from the United States but I'm spending my summer holidays in Medellín. Come and see me in Tokyo in your summer holidays.	I've told you, they're away on their summer holidays. He just came here for the summer on holiday. It was on my summer holidays. I went to Bali on summer holiday. I remember when you were little and we used to go on those crazy summer holidays. We're going to take advantage of the fact that the staff and students are away on their summer holidays to subject the department's physical defences to an exacting, empirical test. No, he's been away on summer holiday. Being back at school on the summer holidays is just wrong, innit?

in summer holiday

11,520,000 results on the web

on summer holiday

18,600,000 results on the web

More popular!

Do come and see me in 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.
Come and see me in the summer holidays.
In doing so
He proposed bringing the elections forward from June to May (in order to avoid summer holidays in Northern states), and to truncate the time for polling itself to two days maximum.
Mrs Theorin referred to what we in Sweden call summer holiday circumcision, which is when girls are taken abroad to be circumcised.
Actually I'm from the United States but I'm spending my summer holidays in Medellín.
Come and see me in Tokyo in your summer holidays.

I've told you, they're away on their summer holidays.
He just came here for the summer on holiday.
It was on my summer holidays.
I went to Bali on summer holiday.
I remember when you were little and we used to go on those crazy summer holidays.
We're going to take advantage of the fact that the staff and students are away on their summer holidays to subject the department's physical defences to an exacting, empirical test.
No, he's been away on summer holiday.
Being back at school on the summer holidays is just wrong, innit?