These both have the same meaning, although I think the first one is more commonly used:
On Saturday morningS I usually get up late.
On Saturday morning I usually get up late.
These are both correct, but they have different meanings:
My father was happy because he played golf in the morningS.
My father was happy because he played golf in the morning.
The first means that he played golf every morning, or most mornings. The second one is speaking of a single time he played golf in the morning of the day when the sentence is spoken.
The difference is that the first two sentences used the word "usually".
CAN WE SAY "In the morning" or "On Saturday morning" without "S" if the idea is "every morning"?
Yes, but then we have to add something else to get across the idea that it was every morning, and not just a single morning.
- Every Saturday morning, I ride my bike into town.
- On Saturday morning, I always get up early.
- On Sunday, I like to watch football.