I'm a Minnesota girl, but wherever I have lived (Minnesota, Virginia, or London) I have noticed that everyone has a different threshold at which they transition from boots to sandals.
40 degrees: Put away the UGGS. It's time for bare legs + boots (see the rules on this below).
50 degrees: Bring out the peep-toe shoes, open-toe heels, and running shoes (it's running weather). Heck, if the snow was melted I would wear flip-flops and sandals.
60 degrees: Every type of shoe is acceptable as long as it's not meant to be worn in Alaska.
70+ degrees: If you don't want stinky toesies, put those boots away asap. There is nothing worse than seeing someone's feet trapped in boots when its a hot day.
90+ degrees: It's officially gross to wear cowboy boots, EVEN at country concerts. People will assume you're stomping around in puddles of your own sweat. In fact, you probably would be!
COMMENT BELOW: When do YOU think it's okay to start wearing flip flops???