Series 5 began filming in the second week of March, they are now filming the 4th, 5th and 6th episode.
It will most likely air in October due to Doctor Who again being aired later instead of late spring, however if Doctor Who does not air early Autumn then Merlin will air in September. :)
The 29th September in the UK,sorry i don't know for the US. A preview of Merlin series 5 will be shown at Comic Con on July 15th along with a blooper reel, behind the making of the Merlin facebook game and a peek at a YouTube Merlin channel that contains behind the scenes and deleted content from all 4 seasons :)