The weather in Marseille, the capital of Provence, is almost always pleasant, and it’s possible to take one of the city’s many walking tours year-round.
The summer months are mild, so that’s when Marseille is at its most festive and most crowded. Bastille Day celebrations stretch on for two weeks during July.
If there’s a less desirable time to visit, weather-wise, it’s probably January; the average high is 51 degrees.
Note that the Mediterranean waters in Marseille can be crystal clear or terribly polluted, depending on the weather and the winds, but this isn’t a place to loll on a beach. It’s too vibrant a city, and there’s much too much to see.