If it's cloudy then any time would probably be fine. If it's sunny then evening might be better but it only makes a few hours difference, they'll still have to deal with the sun if it's there the next day. If you can disturb the roots as little as possible, and the weather is not too hot sunny or windy they should be OK. If you can time it to plant them before a sequence of several cloudy, drizzly, cool but not cold days that would be a perfect time.
