It wasn't even hot yesterday at around 1pm. It was only 72 but not a garter to be found anywhere. Same spot after 5pm came to life, with dozens of snakes out foraging. They're active all day in the same spot if it's overcast. I think even when it's not hot, they try to avoid the intense midday sun when the UV index is high. This behavior varies dramatically depending on temperature, cloudy, sunny, etc so you'll have to try different times of the day and figure out when it's best at your location.