生日在几月几日用in还是用on为什么

用on,因为具体的某一天要用介词on,in一般是用在年份和月份和季节前

英语日期前介词的使用:若指在哪一年或哪一月,则用介词in,若具体到某一天,则需用介词on。如:

①She was born in 1989

②She was born in August.

③She was born in August 1989.

④She was born on 2nd August, 1989.