This is a totally idle speculation at the moment but I've never know when you should tell a prospective employer about these things. Occasionally I've seen a section on a job application about booked holidays but otherwise I wouldn't know.
I'm applying for 2 jobs tomorrow and have 2 holidays and a smal day l op booked. Am currently working in schools so these are all in school holidays. I probably won't even get an interview but I was just wondering.
If and when you are successful, fingers crossed (!), you will undoubtedly receive some paperwork for "new starters" where it mentions annual leave requests. That could be the prompt you need to mention any existing holiday commitments.
The only time I would mention it any sooner is if you have holiday coming up which is really close to your start date.