Tomorrow is the middle of summer, which began 45 days ago, and will end 45 days from tomorrow (amounting to 91 days per season, and 4 seasons totalling 364 days, which is Good Enough For Government Work).
My rationale is simple: after tomorrow, The Longest Day of the Year, it will not be getting more summer-like, which you might reasonably expect if it were the First Day of Summer.
Which it is not.
Days will shorten for the next 180 days until December 21st , also known (in my universe) as Midwinter.
After which, it will proceed once more to get more summery, until June 21strolls around once more, and I rant again.