Valley of Flower trek is very beautiful and one of the popular trek in Uttarakhand. I suggest you to plan a trip in July or August, In that time you can view various blooming colourful flowers at VOF.
Dear fellow traveler, the valley of flowers is a monsoon trek. The Valley of Flowers is only open from the beginning of June until the beginning of October, as it's covered in snow the rest of the year. The best time to visit is from mid July to mid August, when the flowers are in full bloom after the first monsoon rain. But I advise you to check weather forecast and route properly. Before making your plan.