Don't get angry if people splash water on you as it's the Songkran season.
Don't get angry if people splash water on you during the season, as it's the culture to do it regardless of rank or seniority.
Don't get angry
怒っちゃダメ
if people splash water on you
皆が水をかけてきたら
as it's the Songkran season.
ソンクラーンの期間だから
(それがソンクラーンだからの意味も含めて)
無礼講だから、を明示的に言うなら、2番のように
as it's the culture to do it regardless of rank or seniority
地位や年齢に関係なくそうする(水をかける)のが文化だから
と説明するのもありかと思います。