I will provide you with an apron and a recipe booklet, both yours to keep. I encourage you to bring comfortable shoes for working in the kitchen, and for walking on cobbled streets.
April and May temperatures are mild to warm, and vary between 50 and 75 degrees Fahrenheit. Your best plan is to wear layered clothing so that you can accommodate warm, sunny afternoons and cool mornings and evenings. A water-proof outer garment is highly recommended as is a small umbrella, though I have several to loan—showers are typical.
September is typically warm and bright. The weather is generally more predictable, with temperatures in the mid to upper 70's mid-day, and a bit cooler in the morning and evening.
October temperatures in northern France can vary from very cool mornings (50-55F) to warm afternoons (70-75F). Skies are generally clear and sunny, the air invigorating. Layered clothing is recommended.