Vichy is emphatically a summer city. The thermal season runs from April through October, and the city is at its most alive — and most crowded — between June and September, when the cure visitors fill the hotels and the parks, the opera and casino are in full swing, and the lakeside parks become the social centre of the city. Outside the thermal season, Vichy is quieter, and a number of hotels and restaurants scale back or close entirely.
Spring (April and May) is a good time to visit if you want the thermal infrastructure open but the city less busy. The chestnut trees in the Parc des Sources are at their best in May, and the weather is mild enough for the river and lakeside parks to be enjoyable. Autumn (September and October) is similarly pleasant, with the opera season still running and the thermal crowds beginning to thin.
Winter visits are possible but limited. The thermal baths operate year-round, but much of the resort activity shuts down, and the city can feel strangely empty without its summer visitors. That said, the Belle Époque architecture reads differently in a grey November drizzle — more melancholy, and arguably more honest about the city's complicated history.
The Vichy Jazz Festival in November is one of the reasons to visit out of season: a well-regarded event that brings serious jazz programming to the opera house and various other venues. The thermal season opening in April is a less publicised occasion but gives the city a pleasantly festive start-of-season atmosphere.