Предлоги времени at / on / in

at six o'clock
at five past two
at midnight
'At' generally refers to exact (clock) time.

on Thursday
on September the tenth
on Christmas Eve
'On' generally refers to a particular day.

in August
in the spring
in 1972
in the 19th century
'In' generally refers to longer periods.

Note, however, that there are a lot of idiomatic expressions of time, e. g. at once, at last, at breakfast, at the weekend, at night, in the daytime (but during the day), in the morning, in the end, on time.