Knysna is on the south coast of South Africa, which means a temperate climate that avoids the extremes of the north. Summer (November to March) is warm and sometimes wet — the southern Cape receives rain year-round but summer afternoons can bring heavy downpours. Winter (June to August) is mild by most standards, with temperatures rarely dropping below 10°C, and clear, crisp days are common.
The Knysna Oyster Festival in July is the biggest event on the town's calendar, drawing visitors from across South Africa. Accommodation in town gets booked solid and prices rise; book well in advance if you want to be there for it.
The Garden Route is busiest at Christmas and Easter, when accommodation everywhere fills and traffic on the N2 builds. If you can visit in March–May or September–October you will have better prices and fewer people, with the weather still good.