I've seen this happen, where stations have a set time to update their playlists. I know a lot of Entercom stations do this once or twice a year (99.7 The Point in KC does this 1-2 times a year); I'm not just talking about adding one or two different songs, but rotating a group of songs in and out. Are there stations you are aware of who have kind of a habitual schedule of updating their playlists?