There are many English and Irish customs about Michaelmas, 29 September or Old Michaelmas Day, 10 October.

Across the British isles it was believed, and is possibly still believed, that eating blackberries after the 29th September was a bad idea. In the Western Antiquary of 1882, it is reported that: Whilst walking the country around [Exeter], a young friend who was with me warned me against picking any blackberries: Because’ said he, grimly ‘it’s past Michaelmass Day and the Devil’s been at 'em." Traditionalcustoms

Traditions from Sussex and Hertfordshire say the Devil is abroad on 10th October, and from Worcestershire and Norfolk where it’s 11th October. That’s a difference of eleven days (inclusive or exclusive) and if you know anything about the English folk calendar, this figure will ring a bell, because 10th October is Old Michaelmas Day. Folklorethursday

Colum Sands has a song which mentions blackberries and September, though not the Devil, on his latest CD, Song Bridge. Before Winter Sets In;

September looks out as the evening draws in,
Her colours are coaxing the spiders to spin.
They'll lay out their lace in the dew of the whin,
For they've plenty to do before winter sets in.

And down the long lane with many's the turning,
Back-to-school children are laughing and learning.
We'll follow the field to the blackberry hill,
See the black, red and green and the blossom there still.

---

BEFORE WINTER SETS IN by Colum Sands on Bandcamp https://columsands.bandcamp.com/track/before-winter-sets-in-2