I looked for a post with the usage of prepositions on dates, months and stuff like that but I couldn't find it. I think this is a very difficult subject and I would be very pleased if somebody can explain us better! (or post the link to get it... )
On February 1996
and so on...
On a day-- On Tuesday, on January 18th
In a month, year, century, etc -- In June, in 1996, in the 21st century