Dear Indrajith/desha1547:
"A Comprehensive Grammar of the English Languge", 2010, Quirk et al, Section, 934, Pages 687/688.
PREPOSITIONS of TIME, ´at; on; in; by ´.
"... there are only two dimension-types, viz ´point of time ´ and ´period of time ´.
´At ´ is used for points of time (chiefly clock-time [1]) and also, idiomatically, for holiday periods [2].
[1] at ten oclock; at 6.30 p.m.; at noon.
[2] at the weekend; at Christmas; at Easter.
The reference is to the season of Christmas/Easter, not to the day itself.
´At ´ can be used for periods when conceived of as points of time, as in
at the time; at that time; at breakfast time; at night.
´On ´ is used for referring to days as periods of time:
on Monday; on the following day; on May the first; on New Year ´s Day.
Also in the expression:
Trains leave the station on the hour. [ ´hourly ´, ie 1 o ´clock, 2 o ´clock, etc.]
´In ´ or, less commonly, ´during ´ is used for periods longer or shorter than a day:
in the evening; in summer; in August; during Holy Week; in 1969; in the eigthteenth century; in the months that followed.
So, FrauSue, Yanogator, Professor Quirk and his 5 colleagues, and I, are all in agreement.
"I sleep at night."
Les Douglas