Whether the game provides prompts or not does not obviate you from being a GM and knowing the various important dates and rules. You screwed up as GM. I'd say take your medicine, learn from the mistake, and move on. I don't expect you'll be overlooking the significance of Aug. 31 again!
Indeed, the problem seems to have arisen because you were too reliant on the game to remind you of everything. Which to me argues perhaps the game ought not to do as much hand-holding of its users.