Optimal:
I would guess, between 10 am and 2 pm, to have maximum foragers away from the hive.
Maybe 70 or 75 degrees...Don't think hotter is a problem for the bees, but it can be for the Beekeeper. :-D
Sunny, not cloudy
Calm, not windy
When I don't have optimal, I pretty much go with, if they are flying, it's OK to go in the hive. However, I understand there mode may reflect the current situation and they may make there opinions known... so dress appropriately. :-D