The best time to visit the Har ki Dun trek is during the summer and post-monsoon months, from April to June and then again from September to November. During these periods, the weather is pleasant, with clear skies and moderate temperatures, making it ideal for trekking. The summer months offer lush greenery and blooming flowers, adding to the scenic beauty of the trek. The post-monsoon months provide clear views of the surrounding mountains and valleys. However, it's best to avoid the monsoon season from July to August, as the region receives heavy rainfall, increasing the risk of landslides and making the trek challenging and unsafe.