The main events of The Wedding of River Song begin and end on 22nd April, 2011 - at 5:02pm. All of time is happening at once, but at that moment in time.
It is reasonable to assume that, following the events of the episode, time and history are restored to normality and that life resumed for Amy and Rory from the point at which this adventure began.
However, there is arguably a gap between the conclusion of these events and the garden scene which serves as an epilogue. River has not accompanied Amy and Rory to their garden following her own part in the episode's events. River's timeline does not run parallel with the Doctor's, and she explains in the garden scene that she has just come from the events on the Byzantium (The Time of Angels). There is no reason to assert that this scene takes place immediately afterwards.
The garden scene, therefore, could have taken place any time between 22nd April 2011 and 25th December 2011, which is the date that Amy and Rory next come into contact with The Doctor in The Doctor, the Widow, and the Wardrobe (Christmas episodes being the easiest to date).
Having said that, River's timeline is also said to run loosely in the opposite direction to The Doctor's. The first time The Doctor met River was her last, and the events of The Time of Angels were, for River, after those in The Wedding of River Song, as she was already his wife. She could therefore have made it a point to visit Amy at this point to explain what had just occurred, and so arguably it could have been sooner rather than later in the possible time period. The fact they were also sat out in the garden at night would suggest it was not winter time, therefore nearer to the April date than the December date.
