1 For me, I would define it by when the ceiling fans get turned on.  Pretty much the same concept, they run all spring and summer, and get turned off in the fall and winter.  For me, that was Friday. 

It's interesting that this weekend was hot enough that I changed out for summer sheets and blankets, turned off the heat, opened all the windows, and turned on the fans, and still had trouble sleeping at night due to the heat.  And yet last night the heat was back on and I had a blanket back on the bed.  I think it will be another few weeks before the temperature is reliably warm.

2 It's the first day of summer here: 95°, and the air conditioner's on.

3 Meanwhile, Brickmuppet is getting snow.

4 By this standard, it was summer almost a month ago here at Pond Central, when I turned on the a/c because it was 85 outside, and nearly as warm inside. 

Summer was short-lived, because two days later the high was 35 and I turned on the heat again.

5 I never even turned on the heater this "winter" just outside DC, but I think it was only partly due to the warm weather.  There are a lot of foreign diplomats in my apartment building, and whoever is renting the unit beneath mine must be from a tropical nation.  Even on the coldest of nights, my bathroom floor was toasty warm all season due to the heat radiating from below.