"he says the Devil spits on them at Michaelmas"
is the Christian feast of St. Michael the Archangel
, and occurs on the 29th of September. It is a version of the Celtic equinox
festival which marks the beginning of autumn. Far back in the past, it was observed on the 10th or 11th of October instead; this is now known as Old Michaelmas Day. It was believed to be the last day of the season for picking blackberries, as the Devil was said to spit on them in his rage at being expelled from heaven, the anniversary of which was also thought to fall on this day.
More about Michaelmas.