Foshan is comfortable between October and April. The summer months are hot and humid in the standard Pearl River Delta way, and the Dragon Boat Festival in June brings the best of Shunde's river culture. The Zumiao area hosts lantern festival celebrations during the first lunar month (January or February) with traditional Foshan paper-cut lanterns, which the city has been making for centuries.
As a practical day trip from Guangzhou, Foshan is accessible any time of year. Allow a full day for the Zumiao complex and Shiwan area, and a separate half-day for Shunde if the food and Qinghui Garden are on your list.