A late warm patch is now often called an ‘Indian Summer’. This term was first used in the United States in the 19th Century and may refer to a warm period in which Indigenous Americans could continue hunting.
Previously, in England, a warm patch in the Autumn was called a ‘St Martin Summer’ or ‘a Halloween Summer.’ St Martin’s Day is the 11th November, and is, in a normal year, the day around when the weather turns to feel wintry.
A warm spell is, in fact beneficial to many plants. The problem comes if it is followed by a quick cold spell. Plants need time to harden off to prepare to face cold weather. A warm winter will also allow a lot of insects to survive and so in the summer plants will be adversely affected by a plague of pests.