The weekdays
A week consists of seven days:

Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday

The preposition used with days is ‘on’.
Examples:
On Saturday, I will go to the cinema.
On Monday, I have to work.
He is going to play football on Friday.

:) Ok? - Now...:

The months
A year consists of twelve months:

January
February
March
April
May
June
July
August
September
October
November
December

The preposition used with months is 'in’.
Examples:
My birthday is in May.
We will travel to Australia in December.
In September, Jenny will start her studies.

The four seasons:

spring: March, April, May
summer: June, July, August
autumn: September, October, November
winter: December, January, February

Examples:
A lot of flowers bloom in spring.
In winter it often snows.

:) Aaaaaaaaaaaaaaaaaand one last aspect:
The date

When you write the date on top of a letter of example you write:
Saturday, December 11th 2004 or
Saturday, Dec 11th 2004

When you use a date in a sentence you do as follows:

His birthday is on October 21st.
Christmas is on December 25th.
On January 1st the new year begins.
We will arrive on March 22nd.

You also say:

in 1985
in 2004
in the 19th century
in the twenties

:D Any questions? :D