Online you will find that August is the best time however that is not so. Probably you will find very less flowers during August also. Prefer travelling there in late Sep / Oct to enjoy the complete beauty of that place
The best time to visit the Valley of Flowers is from mid-July to late August, when the valley is in full bloom with a stunning display of alpine flora. During this period, the entire valley transforms into a colorful carpet of over 600 species of wildflowers, fed by the monsoon rains. The trek route is surrounded by lush greenery, cascading waterfalls, and mist-covered peaks, making it a paradise for nature lovers, botanists, and photographers. While the valley officially opens in early June, the floral bloom begins to intensify only by mid-July. By August, the valley is at its most vibrant, offering the most scenic views. In September, the flowers begin to fade, but the skies are usually clearer, and the crowds lessen, offering a quieter experience with golden-hued landscapes. However, trekkers should be prepared for monsoon conditions—trails can be slippery, so carrying proper rain gear and trekking shoes is essential. Also, don’t forget to secure the necessary entry permits from Govindghat or Joshimath before starting the trek.
https://www.himalayanhikers.in/valley-of-flowers-trek/