`The one with the power to vanquish the Dark Lord approaches. born to those who have thrice defied him, born as the seventh month dies . and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not . and either must die at the hand of the other for neither can live while the other survives . the one with the power to vanquish the Dark Lord will be born as the seventh month dies .'
So I guess the assumption was that he would be born the following July.