signs for "on"
on (location) = pointing
on (location neutral) = “on” 
On (as 3D description) = eliminate “on”
on (date) = eliminate “on”
on (event  = “during” 
on (topic) = “regarding”

Examples
on (location-neutral) vs on (location)
They danced on the stage.

on (as 3D description)
The drink is on the table.

on (date)
I was born on June 28. 

on (event)
You will open presents on your birthday. 

on (topic)
We had a discussion on history.