A pairing week typically has an overall theme to it, though that's not always necessary. There will be a list of prompts - sometimes there can be even two separate prompt lists to choose from - and there will be a prompt for each corresponding day of the week.
For example, here is the info post
of the last SessKag week held two years ago.
The point is simply to create, based on these prompts: fics, fan art, graphics, playlists, whatever you can think of.
On Tumblr, the week is assigned a tag that should be used, so the hosts of the event can find the works better and reblog them all into their blog so that everyone can find them in one place.