Every summer we get dates flying about as to when the transfer window actually opens and every season there are lots of guesses. July 1st is a popular one, as is June 1st but neither are correct.
I wasn't actually sure when I asked the question, which is why I asked, but I got an answer from the FA Premier League.
That answer is below and I'm going to refer people to this page whenever someone comes on and says the window opens on a date other than the content of this email, because this is what I have been told and until there is an announcement from the FA Premier League to the contrary, this is what I'm going to use.
FA Premier League
The first Transfer Window in any year shall commence at midnight on the last day of the season and shall end on 31 August next if a Working Day, if not, on the first Working Day thereafter, at a time to be determined by the Board.
The second Transfer Window in any year shall commence at midnight on 31st December and shall end on the 31st January next if a Working Day or, if not, on the first Working Day thereafter, at a time to be determined by the board.
So, I hope this puts to bed the debate on when the transfer window actually opens. For the record, July 1st is the date you can begin to register players, it has nothing to do with when you can actually start making offers and offers can be accepted.