In the following blank, we should fill in At or On, and what is the difference?

Chinese people celebrate the Moon Festival. ______ ( At or On) that night, families will get together, eat moon cakes, tell stories,.........

Thank you for the help.
ON that night. 'At night' is idiomatic; if there is a modifier, 'at' no longer appertains.

if there is a modifier
A premodifier or postmodifier? Now can we say something like this?:

-At the night (not THAT in this case) named he anchored his boat at the wharf.