Looking on the internet deeply has found these results:

I will be on holiday is the most popular phrase on the web.

I will be in holiday 1710,000 results on the web	I will be on holiday 11,890,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
The global economic crisis has caused a general fall in tourist numbers, in nights spent in holiday accommodation and in the turnover achieved by sectoral operators. A vote for this package is a vote for consumer confidence in holiday products, for clear consumer information, and for a competitive and responsible holiday industry. This rule applies in particular to notifications transmitted at weekends or in holiday periods. Well, I will have to remember to clarify that In our holiday newsletter. Moreover, boating activities often take place at the weekend and in holiday periods resulting in a concentration of the nuisance they can cause to local inhabitants. On Sunday, 23 July 2006 the families' three sons were the victims of an unprovoked attack by eight Croatian youths who beat them up with knuckledusters in a swimming pool in holiday park. I will start with a holiday.	I'm going on holiday for a week. I don't talk politics on holiday. No, I'm on holiday. The caucasians have all gone on holiday. I hack airline companies, look for people going on holiday, stay at their houses. Perhaps the professor went on holiday. I'm getting a jump on holiday shopping.

I will be in holiday

1710,000 results on the web

I will be on holiday

11,890,000 results on the web

More popular!

The global economic crisis has caused a general fall in tourist numbers, in nights spent in holiday accommodation and in the turnover achieved by sectoral operators.
A vote for this package is a vote for consumer confidence in holiday products, for clear consumer information, and for a competitive and responsible holiday industry.
This rule applies in particular to notifications transmitted at weekends or in holiday periods.
Well, I will have to remember to clarify that In our holiday newsletter.
Moreover, boating activities often take place at the weekend and in holiday periods resulting in a concentration of the nuisance they can cause to local inhabitants.
On Sunday, 23 July 2006 the families' three sons were the victims of an unprovoked attack by eight Croatian youths who beat them up with knuckledusters in a swimming pool in holiday park.
I will start with a holiday.

I'm going on holiday for a week.
I don't talk politics on holiday.
No, I'm on holiday.
The caucasians have all gone on holiday.
I hack airline companies, look for people going on holiday, stay at their houses.
Perhaps the professor went on holiday.
I'm getting a jump on holiday shopping.