Rennes is the capital of the Brittany region of France.
Brittany is a peninsula between two cold bodies of water, the North Sea and the Bay of Biscay, so summer heat is moderated by coastal winds.
Rennes offers moderate summers, good if slightly nippy and wet fall weather, and an extremely mild but wet winter.
There's really no "bad" time to visit Brittany, as long as you don't mind the cooler and changeable ocean climate. Rennes is inland in Brittany, so the average temperatures are warmer by about 10 degrees compared to coastal cities.
Summer is a great time to visit Rennes. Expect some rain any time, but rainfall is lowest in August. Spring is also a decent time to go to Rennes. Many visitors recommend late September, when the tides recede and you can wander out on to the exposed sea floor.
The chart below shows the historic high and low temperatures and average monthly rainfall for Rennes.
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Le Florin (value)