Taking a cue from Twitter, a number of photographers have started to add hash tags to their photos based upon the days of the week. For example #MountainMonday and #ToyTuesday. These tags can be searched from Google Plus and I believe a similar project is underway at Facebook.
The themes are curated by Google Plus users which give them a bit of structure. #WildlifeWednesday is curated by Mike Spinak.