If I make it very specific, then first week of March is the best time to visit Auli.
Let me explain why.
This is the time to enjoy full snow in Auli with very little cold compared to December and January.
This is the time when you can find Hotels in Joshimath are more cheaper due to less rush.
This is the session of sky sport and tournament in Auli.
So this is the perfect time for Family trip to Auli.
I love the first week of March because of my photography hobby. The beauty of Himalayan mountain peaks at its best this time due to crystal clear atmosphere. Thus, this is the best time to bring back best memories in the hard drive.
View from Auli is not such clear even in summer. Thus visiting Auli after March is not a good choice.
However to experience a snow fall, you can visit Auli from December to February. But roads are also closed many time during peak winter.
Thus best time to visit Auli is from mid February to Mid March.
So plan your next trip to Auli accordingly.