5.95 'on' for short periods of time

If you want to mention the day when something happens, you use 'on'. You can do this with named days, with days referred to by ordinals, and with days referred to by a special term such as 'birthday' or 'anniversary'.

You can use 'the' with named days for emphasis or contrast, and 'a' to indicate any day of that name.

You also use 'on' with 'morning', 'afternoon', 'evening', and 'night' when they are modified or qualified.