Uttarakhand has no single best season, it depends entirely on what you want. Char Dham, snow, flowers and tigers each have their own window. Here is the honest month-by-month truth.
If you want one month that does the most: October.
Jan-Feb: Deep winter. Snow in Auli and Dhanaulti. Char Dham closed. Rishikesh cold but peaceful. Best for snow and quiet.
Mar-Apr: Spring. Rhododendrons bloom. Hill stations wake up. Char Dham opens late April/May. Lovely all round.
May-Jun: Peak season. Everything open, Char Dham in full swing, plains escapees fill the hills. Great weather, heavy crowds, book far ahead.
Jul-Aug: Monsoon. Valley of Flowers blooms, the one reason to come. But landslides close mountain roads and ground helicopters. Genuine risk. Travel only with buffers.
Sep-Oct: The sweet spot. Monsoon clears, air sharpens, crowds thin, Char Dham open till Diwali. Best all-round window.
Nov-Dec: Crisp and clear, cold nights. Char Dham closes around Diwali. Excellent for hill stations and views.
This deserves its own section because people underestimate it. July-August in the Uttarakhand mountains is genuinely risky. Landslides close highways for hours or days. Helicopters ground. The 2013 Kedarnath disaster was a monsoon cloudburst.
The Valley of Flowers only blooms then, so if that is your goal, go, but build buffer days, get insurance, and accept the schedule may break. For Char Dham, we advise against it.
If you have flexibility, come in October: Char Dham still open, mountains at their clearest, hill stations pleasant, crowds gone, roads dry. Nothing else does this many things well at once.
Second choice: late September or March. Tell us your goal and dates and we will tell you honestly whether your month works, including when the answer is 'move it'.