Fortnite: Battle Pass validity?

So I would buy the Battle pass, but my brother told me that it expires every month. So my question is, if I buy the pass today, will it expire on the first of the month or from the purchase for a month?

The pass usually lasts 1 month as long as the season

The Battlepass lasts until the end of a season, I think one lasts in about 2 months. The Battlepass menu also shows how long the Battlepass is still up to date / when the new season begins

Attention i would only buy a pass at the beginning of a season

Not every month! But I can't tell you when it expires D