When to tell a potential employer that I won't be available for 3 weeks?

Hello all,

I have an interview today and if I get the job, I would like to tell my employer that I won't be available to work over winter break due to prior family commitments. How do I bring this up with them? Do I tell them as soon as I get the job offer (if I get the job)? Or do I tell them a few days/weeks into the job?

Any advice would be great!

Edit: Not sure if that is the right flair because there is no "advice" flair

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