Hey, I'd say between March to June, I went in the month of April and it was the right decision. The view from the top was exactly the way I wanted it to be, snow capped with perfect weather. The trek starts from Galu Temple, Dharamkot and it takes around 3-4 hours to reach the top, if you are not planning to stay there I'd advise you to leave early otherwise you can start around 11 and camp at Triund Top. September and October is also pleasant to visit Triund.
The Trek from McLeodganj to Triund is a sharp one of over 1100 meters and it is absolutely worth it because of the refreshing walks amongst the Rhododendron and Oak Forests. The months of May-June & September-October are the best time to visit this place as it is very pleasant here during these months.