The Summer solstice was last Thursday. Most people know that means the longest day of the year is essentially the same day. So that might led one to believe that the day the Sun sets latest is the same day. Logical. Yet incorrect. The day of latest sunset for most folks in the Northern hemisphere is actually this coming Wednesday, the 27th. Why? The short answer is that because the Earth's orbit is not a perfect circle, so the Earth speeds up and slows down in its orbit. That has a small effect on when the Sun rises and sets. (Obviously the speed of revolution is the major determinate here, but the Earth's speed of revolution is extremely constant.)
Most of us measure time with a clock, but if we were using the Sun and real noon -- when the Sun is at its highest point in the South (for the Northern hemisphere) at a given location -- the apparent anomaly would disappear. One paying that much attention might also notice that the elapsed time from one noon to the next is not constant. The interested reader may find additional clues about these anomalies in "Where's the Sun?", which I posted earlier this year.
The upshot of this for me: I think this week is the best time of the year to throw a party outside, if you want the maximum amount of time before it gets dark. :) The difference from day to day (this time of year) is mere seconds, so it doesn't actually make a whole lot of difference, but it's the idea that counts. :)
There's another thing one can consider: If we're just talking about natural light outside, there's an additional possible source of illumination: The Moon. The Moon's cycle is essentially independent of the Sun's, so the day of latest sunset might occur at full Moon, new Moon, or anywhere in-between. This year -- this week -- the Moon moves from first quarter (actually last Thursday, the 21st) through full (on Saturday, the 30th). The first quarter Moon always rises at about solar noon (11 am for us now, since we're in daylight savings time), and is at its highest point about sunset. The full Moon always rises at about sunset, and is highest at midnight. So the Moon may be of some help for early evening illumination for the next several days.
Of course if it rains you're screwed. :) Go talk to the weatherman about that. And stock up on gin and tonic if you're having a party, because it's definitely G&T season.
For further ephemera like these, you're in luck: As I think of them, I add them to the Anti-Calendar. Knock yourself out. :)