Without a doubt, the best time of year to visit Lille is in the spring. The beauty of France in the spring is unmatched anywhere else in the world. All of the flowers are in bloom, and the smells and sights are magnificent. Lille also has some great outdoor activities during the spring. Many of the museums, including the Museum of Fine Arts, have many outdoor exhibits at that time. Many of the works they show are rotated between different museums in Europe, and many of the pieces can only be seen during that time of year also. Another option is to visit during the summer. The weather is a bit hotter, but you will still have good weather overall, and you will be able to do many outdoor activities. If you visit during the fall or winter, it will obviously be colder and you might not be able to do all of the activities you had hoped to do so. France can have milder winters than we experience in the United States, but it is still cold and they can get big snow storms as well. If you enjoy the colder weather then this is a good time to visit, but for the maximum amount of acitivies and things to see in Lille, it is best to visit during warmer months.