at, on, in, from.

at + hora......................................I wake up at 7.30
on + día de la semana..........................On Mondays, Friday afternoon.
in + mes o periodo del día (exceptuando "night", que se dice "at night")..............in August, in the morning.
from + periodo de tiempo.....................................from noon to midnight, from 7.30 a.m to 2.30 p.m


Más información: 
http://www.englishclub.com/grammar/prepositions-at-in-on-time.htm