The warm breeze coming in off the North Sea keeps Antwerp pretty temperate all year around. It rains throughout the year, but less so in the summer. There's no ideal time to go. Go when it suits you best, or if there's an event you'd particularly like to catch. The sun is pretty regular in the summers and doesn't blaze down. This is also festive time for Antwerp, but it's also the peak tourist season. It's crowded and things are much more expensive. There's not much to do in winter.