是on Mothers Day.英语时间的用法为:in年in月in季节，如in 2023, in May, in summer；at加时间，如at 6six oclock, at half past ten； on在详细日子前，如on Monday, on June second, on Teachers Day。

“在母亲节那天”是详细的日子，故此，要用on。差不多节日都是用ON 的，，，，也有例外，春节，at Spring Festival英语"祝你母亲节快乐"？肯定是Happy Mothers' Day! 因为mother 是全部母亲，故此，应该Happy Mother's Day Happy Mothers' Day 母亲节快乐！