Lille works year-round but the most compelling reason to time a trip is the Braderie de Lille on the first weekend of September — the largest flea market in Europe, when the entire city centre becomes one vast outdoor sale and every restaurant competes to build the highest mound of moules-frites shells on the pavement outside. Book accommodation months ahead for this weekend.
Spring and summer (April–August) are the most pleasant for café terraces and walking Vieux-Lille. The Citadelle park is particularly enjoyable in warm weather. Winter is cold and often grey — this is northern France on the edge of Belgium, not the south — but the Christmas market on the Grand'Place is genuinely good and the museums and estaminets are excellent reasons to come regardless of weather.
The Fêtes de Lille in June celebrate the city's Flemish heritage with parades, concerts, and folk events centred on the Grand'Place. The Lille Piano Festival in October is one of the significant classical music events in northern France.