I usually do it around 6:00pm so everything is lit up but it's not too dark outside. or 5am when it's quiet everywhere and it would be even better if it's raining
I update my town usually when the sky is purple. That's usually around 7pm. Sometimes, I do early mornings like 4am just for the quite peace weirdness of the hourly music. I love updating during snow and rain too.
usually when its shortly passed sunset, kinda like what agscribble said. i remember once it was a clear day all day, and the sunset was one of those deep orange/red ones that happen on the island. really wished i updated it because it was so beautiful and apparently it rarely happens. kinda wondering if anyone has this sunset saved in their dream town?