Well, the past few years it's been February, May, August, and November, with very little variance on that for the big sets. The release of Dragon Vault last fall in October was an exception to that, though, but that was a mini-set, and the initial release of the first Black & White set was delayed about a month to correspond to the release of those games.
I'm not sure on the exact date yet, but it'll probably be around mid-August.