(While i have no "source info") my guess is that the editorial calendar is wrong.
* D&D books seem to always be released on the third Tuesday of every month (though, i don't watch it closely, i believe that is the "system")
* Amazon still says 4/21 (although , admittedly, amazon is not a great authority on such matters)
* The 'slot' it has on the calendar just happens to be the exact same space from the march calendar (i.e. first space, third row)... so (and it's happened before) i think it was left on the same space in the calendar not realizing it was supposed to shift down a week.
that's all weak evidence. but it's all evidence none the less in favor of saying that the site calendar is not correct right now.