The coldest winter I ever spent was a summer in San Francisco.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]
(Spurious)

Frequently attributed to Twain, but undocumented in any of his writings. The origin of the phrase seems to be in a letter from Horace Walpole to Mary Berry (29 Jul 1789), attributing a quip to the English actor James Quin:

Quin, being once asked if he had ever seen so bad a winter, replied, “Yes, just such an one last summer!” -- and here is its youngest brother!

Twain, in turn, mentioned the observation in a letter to Lucius Fairchild (28 Apr 1880), using it to denigrate Paris, France:

For this long time I have been intending to congratulate you fervently upon your translation to -- to -- anywhere -- for anywhere is better than Paris. Paris the cold, Paris the drizzly, Paris the rainy, Paris the Damnable. More than a hundred years ago, somebody asked Quin, "Did you ever see such a winter in all your life before?" "Yes," said he, "last summer." I judge he spent his summer in Paris.

When "coldest winter ... summer" phrase first achieved popularity in that form (around 1900 or earlier), the targeted city was Duluth, Minnesota, followed by other cities in Minnesota and Wisconsin, before being grafted onto San Francisco and, again, Mark Twain.

More discussion about this quotation:

• “The coldest winter I ever spent was a summer in San Francisco,” - Said Mark Twain?
• The Coldest Winter I Ever Spent Was a Summer in San Francisco – Quote Investigator