The best time to visit Grevenmacher is late summer and autumn. Cycling along the Moselle is at its most pleasant from June through October, and the vineyards are in their full glory from August onwards. The harvest (vendange) runs through September and October, when the wine villages along the river are particularly lively.
The Grevenmacher Wine and Grape Festival (Fête des Vendanges) takes place every September and is the main annual event in town — a convivial weekend of wine tasting, local food stalls, and live music that draws visitors from across the region. If you can time a visit to coincide with it, do. Winter is quiet; the Butterfly Garden stays open year-round and is actually a pleasant escape from the cold, but most of the river-terrace appeal evaporates.