Question

I am going to visit my grandparents on the morning of sunday

Answer 1

morning,noon,afternoon,evening,night这些词只要前头有形容词,或者有后置定语（即只要有定语修饰）那么前面的介词一律用介词on,无论是Saturday或是early.

例如：on Monday morning

on a cold night

on the morning of April 28th.

如果单独出现没有定语,那么是at night,at noon,in the morning.