Above 6.1 C., sunny and calm, for a few seconds just to lay it on and close it back up.
Above 12,8 C. sunny and calm, to look it over before closing.
Above 15.5 C,, sunny and calm, to pull a frame out for a quick look.
Above 21 C., sunny and calm, to inspect the hive thoroughly.
OH, did I mention, choose a day without wind or clouds?