I would say that the only month almost guaranteed to have good weather in Montreal is September.

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September - Cool, temperate, sunny.

October - Usually quite good, sometimes cold toward the 2nd half of month.

November - Usually bad. Almost always cold and grey toward the 2nd half of month.

December - Sometimes okay (mostly because of novelty of winter and Christmas). Other times minus 30 for two weeks fucking straight...

January - Always bad.

February - Always extremely bad.

March - Always bad, with everyone completely fed up and all hoping for a miracle March that almost never happens.

April - Usually bad, but for some reason people seem to expect otherwise. Either still winter, or constant cold rains. If by chance it's good, then it's the best month of the year.

May - The new April. Rain. Rain. Rain.

June - Generally good. Sometimes rain.

July - Generally good. Sometimes way too hot and humid.

August - What is this, Vietnam?!?

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