Post-doctoral positions are usually for one or, exceptionally, two year periods. These positions consist on visiting a given Department without being enrolled in any graduated courses but with the option to do it.
Where to find some information
Most of the information about Post-doc positions will be available at the departments' webpages. However, you may find some of them published in the following websites.
Here is an approximate listings of the most common fellowships. Notice that the exact days may change from one year to another: