why do you go to school ___ Saturdays this year?----
问题描述：
why do you go to school ___ Saturdays this year?----
答
on
on用法：
（1）具体的时日和一个特定的时间,如某日、某节日、星期几等.如：
On Christmas Day（On May 4th）,there will be a celebration.
（2）在某个特定的早晨、下午或晚上.如：
He arrived at 10 o’clock on the night of the 5th.
（3）准时,按时.如：
If the train should be on time,I should reach home before dark.