Note: Again, I'm going with Chronological time here, not "soap opera time." As usual, all events are taken from my timeline. When there are two possibilities for when an event occured in the timeline, I chose the one that I personally favour.
Timelining in the summer months is actually fairly hard - so these are all just approximate events, as I have very little in the timeline that takes place on a firm date in August (especially for Cas).
Castiel's "Birth" - 2007 - Castiel is a wavelength of celestriel intent. As a soldier of the garrison, he watches over earth and tries not to step on important things.
2008 - Castiel fails to rescue the Righteous Man from hell before he (and the first seal on Lucifer's cage) break. He won't find out until later that the campaign was probably sabotaged from the start.
2009 (Summer) - Dean and Castiel try to find Raphael in Maine...and in the process Dean takes Cas to a den of iniquity.
2010 - Castiel returns to Heaven after the Apocalypse has been thwarted and he believes he has rescued Sam - Sadly, Castiel discovers that things are not how he hoped they would be.
2011 (late summer or early fall) - Castiel reunites with Dean and Sam after it seems they have a lead on one of the missing weapons of heaven. Castiel is reunited with Balthazar, a brother he believed to be dead.
2012 (unknown exactly when) - A man stumbles naked out of body of water with no knowledge of who he is. He is found by a woman who feels as though she was sent by God to help him.
2013 - ??
And there you have it...after X number of years of stability, the past 6 years have been a real rollercoaster for poor Cas.