Qingcheng Mountain is classically visited in mist and light rain, and the Taoist aesthetic of the place — concealment, revelation, the interplay of visible and hidden — makes sense precisely in that climate. Attempting to visit only in clear weather misses the point somewhat, though the summit views are obviously better when the sky cooperates.
The busiest season runs from May through October, with weekends and national holidays bringing serious crowds on the front mountain trail. Spring and autumn weekdays are the best combination of manageable weather and manageable crowds. The mountain is technically open year-round; winter brings fewer visitors and occasional snow that makes the forest extraordinary, though the path can become slippery.