One of Europe's oldest Christmas market, and likely the most famous in France, is Strasbourg's. Its existence is attested as far back as 1570, appearing in the years following Protestant Reformation. Today it is a sprawling event, covering all the main squares of the central island of Strasbourg, and it's very busy, especially in the evenings and on weekends.
The traditional Alsatian name of the market is Christkindelsmärik, "the market of baby Jesus", while the city's more recent branding (since the 1990s) proclaims the town as "Capital of Christmas".
Pictures from 2018 - I haven't been to this year's market yet, but I plan to once my workload deflates - I get my annual stock of jams from the local producer's market!