En passant is a chess rules that is: when a pawn is on his home square, moving two squares forward, if an enemy pawn is right beside it, it can take the pawn by moving diagonally to a black square. Like this:
You missed one point.
In the first position, suppose this happens. 1. a4 Kg8 2. Kg1 is played. Here, is 2...axb3 possible? Without that important point, one can claim that en passant can be performed anytime.
En passant is a chess rules that is: when a pawn is on his home square, moving two squares forward, if an enemy pawn is right beside it, it can take the pawn by moving diagonally to a black square. Like this: