Soignies has a typical temperate Belgian climate: mild, overcast, and often damp. There is no truly bad season for a visit, but the experience is noticeably better between April and September when cafe terraces are in use and the days are long enough to walk the quarry landscape in comfort.
Spring (April–May) is the most pleasant time: mild temperatures, occasional sunshine, and the town is not crowded. The countryside around the quarries looks its best when the fields are green.
Summer (June–August) is warm and the Grand Place comes fully to life in the evenings. July can be the wettest month, but rain is rarely prolonged.
Autumn (September–October) is good for walking, with cooler temperatures and the light taking on a lower, more dramatic angle over the stone landscape.
Winter (November–March) is grey and cold, but the collegiate church is no less impressive in winter light, and the town is at its most local and unselfconscious. The Christmas market on the Grand Place is small but genuine.
There are no major events in Soignies that would make one date significantly better than another. The town's Ducasse (a traditional Walloon procession festival) takes place in late August and is worth seeing if you happen to be there.