Saint Luke's summer
According to Oxford Dictionaries, Saint Luke's summer (a British term) is a period of fine weather around the 18th of October, which is Saint Luke's feast day. I honestly don't remember how I came across this phrase, but it fascinates me... an older version of what we'd call "Indian summer." Especially in England and Europe where the days of the year were marked by saints' days and sundry feasts, this makes particular sense. It puts a new spin on the word lukewarm, as well.
It makes for an interesting metaphor - a little pocket of summer come to rest inside another season. For the last several days we've had a glorious St. Luke's summer: the leaves are golden, red, orange and purple, beautiful autumn, and yet temperatures climbed into the 80s. It was summer. I tend to enjoy these bizarre weather-fronts: the odd January days that reach the 60s or 70s and melt a month's worth of snow; the chilly, blustery fronts in July when we suddenly wonder where the oven of summer has gone. They're rogue summers and winters - visiting out of season but welcome guests nonetheless. I don't think it's as significant as climate change, but weather-change and weather-mood.
There is another version of Saint Luke's summer which is Saint Martin's summer, basically nice, warm weather around the 11th of November, the feast day of St. Martin of Tours.