In the U.S.Mother's Day is a holiday 介词 the second sunday 介词 May.

In the U.S.Mother's Day is a holiday 介词 the second sunday 介词 May.
In the U.S.Mother's Day is a holiday 介词 the second sunday 介词 May.

In the U.S.Mother's Day is a holiday 介词 the second sunday 介词 May.
on in

on , of

on of

on吧

on, in

在具体某天前要用介词on（0n Monday）,在月份前用介词in