When is it appropriate to tell a new employer about a holiday/vacation that is already booked? Especially when it is close to the start date of the new job.
If the planned holiday is soon, then you can do it when negotiating your start date. You can cast it as "I have this trip planned; should I start after I get back or can we talk about my taking the time off?".
If the planned holiday is some months away, but will occur before you will have accrued enough vacation time to take it, then bring it up when you're discussing compensation. Tell them about the trip and ask whether you can go negative or take unpaid leave for this trip. I have done this (with the result being "go negative") and I have had many coworkers do it.
If a company wants you to join them, then in the grand scheme of things your two-week vacation just isn't a big deal and they will generally want to work with you to resolve the issue.
Even if you will have enough vacation time built up, it's a good idea to share your plans with your manager as soon as you can.
If you don't mention this before you accept the offer, the likelihood is that the time may not be approved as you do not have enough earned vaction time. If you have already started the new job, it is already too late.
Further, be prepared to take this a leave without pay even if you mentioned it up front. However, in your salary negotiations you should mention it and negotiate for the leave to be with pay. YOu may or may not get that and then you have to decide, but most companies will be accommodating for plans they knew about before they hired you.