Saint Luke's summer

Saint Luke's summer

n
(Physical Geography) a period of unusually warm weather in the autumn
[referring to St Luke's feast-day, Oct 7 in the pre-Gregorian calendar (now Oct 18)]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014