There are three prepositions in English that are used with time in, at, on. Generally, in shows the “largest” time or place, on shows dates and time, and at usually shows the “smallest” time or place.
Let's look at the usage for in, at, on.
|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,
|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.|
Note: Do not use in/at/on before “next” or “last”
We will get married next year.
Their baby was born last March.