Paris has a temperate oceanic climate: winters are mild and grey rather than brutally cold, summers are warm and occasionally hot, and spring and autumn are the most reliably pleasant seasons. Rain is distributed fairly evenly throughout the year, so an umbrella is always worth packing.
Spring (April–June) is the classic time to visit. Temperatures are in the 15–22°C range, the parks are in bloom, and the city is in full daily rhythm without the summer crowds. April can be showery. May is excellent. June starts to fill up with visitors.
Summer (July–August) is busy and can be hot — Paris has experienced heatwaves above 40°C in recent years, and air-conditioning is less universal here than in southern European cities. August is when many Parisians leave for the coast or countryside. Some restaurants and shops close for the month, but the city is quieter and, in a particular way, more relaxed.
Autumn (September–November) rivals spring as the best season. Temperatures are pleasant through October, the tourist crowds thin after September, and the restaurants and cultural season hit their stride. November can be cold and grey but is never miserable.
Winter (December–March) has its pleasures: Christmas decorations on the Champs-Élysées are genuinely impressive, the Marché de Noël runs through December, and the museums are uncrowded. Cold and damp are the main challenges. January is the cheapest month to visit with the most hotel availability.
If your visit coincides with the Paris fashion weeks (late September and late January/February), hotel prices spike and bookings become difficult. Plan accordingly.