Antwerp's weather is North Sea standard — mild summers, cool wet winters, plenty of grey skies in between. There is no bad time to visit; pick by what you want from the city.
Summer (June–August) brings the best weather: temperatures in the low to mid-20s°C, long evenings on the café terraces, and the city's outdoor festivals. The Sinksenfoor funfair runs at the Spoor Oost in late May/early June. Antwerp Pride is the second weekend in August. Linkerwoofer and Laundry Day are the main summer music festivals. July and August can also see sudden heavy showers — pack accordingly.
Christmas and New Year is the second peak season. The Christmas Market at Steenplein and Grote Markt runs through December, with an ice rink, mulled wine, food stalls, and craft sellers. Hotels fill ahead. The lighting on the Cathedral and old town is particularly good.
Spring and autumn are quieter, cheaper, and good for museum-focused trips. April brings the cherry blossoms in the city parks; October brings the leaf colour. Both seasons are good for the long museum visits (KMSKA, Plantin-Moretus) that the city specialises in.
Winter (January–February) is genuinely quiet. The advantage is empty museums and short queues at the Cathedral. The disadvantage is short cold days. Bring layers.