~が (subject of subordinate clause)

Within subordinate clauses (relative clauses, embedded propositions, temporal clauses with 時 / 前 / 後 / 間, noun-modifying clauses), the subject is normally marked with が rather than は. は's topic-marking scope tends to be the matrix clause, so it can't host an embedded subject. 私が行く時 ('when I go'), 雨が降る日 ('a day on which rain falls'), 君が着く前 ('before you arrive').