Tonight may or may not be Midsummer Night's Eve, depending on how the Ancient Celts calculated it, and they aren't around to consult on the question. But they reckoned Summer as running from around May 1 and ending with the end of July. The Summer Solstice, which is our modern First Day of Summer is the longest day in the Earth's Northern Hemisphere.
The Ancient Celts, having no electricity or video games to distract them, tended to notice such things.
So even though the Solstice didn't fall exactly in the middle of their Summer, they knew a tipping point when they saw it. Thus, Midsummer... and Midsummer Night's Eve.
When Christianity supplanted paganism, as it more or less did, eventually, even in Northern Europe, a lot of calendar dates were jiggered to accommodate newly adopted saints... and still preserve the existing holidays. Thus, Midsummer Night's Eve, in some locations, may have become more associated with the Nativity of John the Baptist on June 24.
But, yes, the bottom line is we do celebrate Midsummer Night's Eve tonight... and only the First Day of Summer tomorrow. The Ancient Celts would probably find that amusing.