France has three distinct climate zones and the timing question really depends on where you're going and what you want to do.
Spring (April–June)
The best time to visit most of France. Crowds are manageable, prices are reasonable, the countryside is green, and the weather in Paris is pleasant without being hot. Provence begins to show lavender in June. School holidays in May create some busy periods, particularly around Ascension and Pentecost weekends.
Summer (July–August)
The peak season. Prices spike, popular destinations get extremely crowded, and much of the French population goes on holiday simultaneously. The Mediterranean coast and the Alps are busiest. On the other hand, summer is the best time for the mountains, beach towns come alive, and many festivals take place. Just book everything well in advance and expect to share your favourite spots with large numbers of fellow visitors.
One French peculiarity: many small restaurants, shops, and businesses close in August. In Paris, this can make certain neighbourhoods feel emptied out — which is either peaceful or frustrating depending on your perspective.
Autumn (September–November)
An excellent time to visit. Harvest season brings wine festivals across Burgundy, Champagne, and Alsace. The light is beautiful, crowds thin out after September, and temperatures remain comfortable. Provence is particularly good in September and October.
Winter (December–February)
Cold and wet in most of the country, though Paris and the north rarely get serious snow. The Alps and Pyrenees are at their best — ski season runs from December through April in the higher resorts. Alsace has the best Christmas markets in France, drawing visitors from across Europe. Outside the ski resorts and festive markets, winter travel offers the best value and the fewest crowds.
Practical note: The French school calendar drives enormous seasonal surges in travel. The two-week Christmas holidays, the two-week February winter break, the two-week Easter break, and the long summer holiday (July–August) all produce sharp spikes in domestic travel. If you can avoid travelling when French families travel, you'll have a better and cheaper trip.