The phrase "sandal weather" means that the weather is warm enough for people to wear sandals on their feet instead of closed-toe shoes or boots. Full definition
I am ready for sandals weather and all thing spring and summer. (pinkandnavystripes.com)
Even though it's getting warmer out, it is definitely not mini skirt and sandals weather yet. (seekingsunshine.com)
Unfortunately, it is decidedly not sandals weather in Chicago and I completely forgot to take pictures of the sandals I wore this weekend when it was still warm enough to bare my toes. (looplooks.net)