in后面加的月份或不确定的日子，on后面是确定的日子例如：my birthday is in July my birthday is on July 1st
in后面接年份或月份 on后面j接具体某一天 比如 my birthday is in September, my birthday is on September 19
如果只说年月,用in如果具体到某一天,用on.
