When is the best time to visit Kathmandu durbar square? Probably the morning as you'll see it come to life along with the people of Kathmandu.
Durbar square though rich in history is and a UNESCO site is an active are for morning markets and religious rituals.
In the photograph you can see in amongst the old temples a typical scene of sadhus, market vendors, school children, people going for blessings and people simply walking through the square on the way to work.
Next: read the Nepal Travel Guide
Kathmandu durbar square travel guide
Blog posts about Kathmandu Durbar Square
Watching Kathmandu at dawn - Durbar Square
Kathmandu Durbar Square at dawn