The cool, dry season from November to February is much the best time to visit: warm days, cool nights, almost no rain, and the ponds within the park are still full from the monsoon. March to May is uncomfortably hot — well over 35°C every day — and the lotus ponds dry out. The monsoon (June to October) brings rain and humidity but also dramatically green surroundings; it tends to rain in short heavy bursts rather than all day.
The single best time to come is for Loi Krathong in November (a full moon, usually around mid-month). Sukhothai is one of the traditional homes of the festival, and the historical park is dramatically lit, with sound-and-light performances, candles floating on the ponds, and small flotillas of krathongs released into the moats. Book accommodation months in advance.