It is June the 1st and I say "Happy Summer" to you, though what "summer" exactly is is about anyone's guess.
Culturally we tend to think of summer as the time period between Memorial Day weekend and Labor Day weekend, inclusive. I like that definition.
Astronomically of course summer starts at the summer equinox, or June 20 or 21. I always thought it was odd to start summer on the day that daylight begins to diminish.
In the "weather" or meteorological" view that predominates in many parts of the northern hemisphere, summer consists of the months of June, July, and August. This was always how I looked at it growing up, well, until the school year started to eat up more and more of summer.
What makes logical sense in many ways is to go by average length of daylight, which would mean that the very middle of summer would be the summer solstice on June 21 (midsummer's night), with summer starting 6 1/2 weeks before that and ending 6 1/2 weeks after. We would do the same for winter, putting it 6 1/2 weeks on each side of December 21.
Unfortunately the weather does not cooperate with this approach, since this would have summer starting in early May - barely Spring in some places north of us. We can thank the seasonal weather "lag" for this lack of scheduling cooperation. It takes a while for things to heat up and cool down. This is more or less due to the very high specific heat of water (aren't you glad to know that!). It takes water a longer time to heat up and cool down than about anything else, so, even when air temps are rising due to longer days and more direct sunshine, water absorbs a lot of this heat energy creating the lag in general temperature increases. And since there is a lot more water than land in the world, the "seasonal" association of summer with higher temperatures does not exactly follow the convenient midsummer night's definition.
I'll go with June, July, and August myself.
And by any measure I was a summer baby...