PREPOSITIONS OF TIME

Preposition	Time	Example
At Use at with times	at 12:00	Brad usually eats lunch at 12:00.
Other common expressions with at	at night	They often go out at night.
	at Christmas (during a period of time around Christmas, for example, December 23rd-28th)	We visit our family and friends at Christmas.
	at the age of...	I moved to California at the age of two.
On Use on with dates and days	on January 18th	Pedro's birthday is on January 18th.
	on Saturday evenings	We usually stay go out on Saturday evenings.
	on Christmas day (on December 25th)	We have a special meal on Christmas day.
In Use in + a period of time = a time in the future	in a few minutes	The bus will be leaving in a few minutes.
	in five weeks also: in five weeks' time	I learned to ride a bicycle in five weeks. We will start summer vacation in five weeks' time.
Use in for longer periods of time	in the 1960s	The astronauts explored the moon in the 1960s.
	in the 20th century	Automobiles and airplanes became very popular in the 20th century.
	in December	In December, you will study less.
	in 2010	The Winter Olympics were held in Vancouver in 2010.

taken from: http://web2.uvcs.uvic.ca/elc/studyzone/490/grammar/prepositions-time-in-at-on.htm