Indian summer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A period of mild weather occurring in late autumn.
• noun A pleasant, tranquil, or flourishing period occurring near the end of something.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a stretch of sunny and warm days during late autumn
• noun figuratively the late autumn of life; a late flowering of activity before old age

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a period of unusually warm weather in the autumn