I've done both - started a new job and then told them and told them then started. TBH I think telling them after you start is "fair, as in you being offered a job should have nothing to do with the fact that you're needing a month off in Feb... BUT... I think it is polite to inform them at interview stage.

My employers didn't really care... but I don't work in a job like teaching where there are clear terms and breaks and things.

GL...