Sunday is the 29th of September - also known as Michaelmas. In the middle ages in England, farmers used Michaelmas as a way to mark the change of seasons; It was time to wrap up the reaping and start getting ready for winter.
And, according to folklore, bounty-thorn (the English folk-name for blackberries) need to be picked by Michaelmas because that was the day that Lucifer was expelled from Heaven. Now according to folklore, once he was cast out, Lucifer promptly fell straight into a blackberry bush. A blackberry bush would not make for a soft landing. Lucifer wasn't thrilled with it either. He supposedly cursed the blackberry fruit, making them unfit for consumption. So unless you want to eat tainted blackberries, get them picked before Sunday. And don't forget, blackberries make a lovely pie or crumble.