I live in Vancouver, and we had snow in March, along with rain. But today it's suddenly sunny....
I generally wear what I want when I want. Open toe heels are fine with me if it matches my outfit (dress/gown) and if it isn't freezing cold outside (if the party is indoors). I've worn boots in summer when we get heavey rains, lol. Basically, you shouldn't feel restricted to wearing certain shoes in certain months.... just like the whole "white after labor day" thing is NOT a rule for me.... I'll wear a winter white coat to match my white baby spy if I want to!