Calm sunny days work best for me and the bees. Morning, afternoon, early evening, whatever. I haven't seen an appreciable difference in numbers of bees at home once a nice sunny day has begun. Rick's right about our comfort - when working bees last weekend it was about 2 pm and low 90s. We had a lot of tasks to do, and I was starting to get a bit sloppy toward the end. 10 am would have been much preferable, but not in our schedule that day.