You're gonna be bothered by how cool I am.

OK, my math is quick and maybe wrong, but the OotP cover was March 20, 2003, which was a mere 93 days before the book came out, because that one was in June; 93 days before 21 July this year is April 19.

But the HBP cover was March 8, 2005 -- a full 130 days before the book. If the DH cover were to come out that many days ahead of time, it would have come out yesterday. :)

So history suggests sometime in the next month. I suppose it could actually be April 17 to start their countdown out with a bang. But basically, literally any day between now and then is possible!