Today is St Martin's Day, or Martinmas (or, in some regions, Martlemas). It's not very significant these days, except that in Scotland it is one of two term days recognised by common law (term days are fixed days of the year when payments are made, tenancies start or finish etc). In former times in England it was a day for hiring servants, holding fairs and slaughtering cattle to be salted for the winter.
In the 19th century a Martinmas or St Martin's summer was what we now call an Indian summer (as I mentioned here when talking about another synonym, All-Hallown summer).
If you live in Fenny Stratford, a district of Milton Keynes, a town about 50 miles north of London, St Martin's Day is a bit more exciting than it is for the rest of us, since it is the day that the Fenny Poppers are fired. The Fenny Poppers are six miniature cannon which are fired in memory of Thomas Willis, an eminent English doctor and founder member of the Royal Society, who died on St Martin's Day 1675. St Martin's church in Fenny Stratford is the only church in England to hold a gunpowder licence. You can read more about the custom here.