I'm not entirely clear on the context of this one.
According to the artist's notes, it has something to do with a date scene from volume 4 of Haruhi Suzumiya-chan.

I'm not really sure I understand the commentary box in the first panel either. What is a future diary date? Is it Kyon's recollection of this date at some point in the future?
Does it have something to do with the series Future Diary?

In volume 4 of Haruhi-chan, Kyon and Haruhi (via Nagato's data manipulation) lose a game and must do the bidding of the winners (mostly Koizumi's ideas). The penalty is going on a scripted date and doing lovey-dovey crap (trying on clothes, going to a romantic movie, holding hands) that is pre-scripted.

Needless to say they don't take it very well. This comic pokes more at the fact that Koizumi ends up spending the day with Nagato and Asahina.

The comic, like pretty much all of Haruhi-chan is pretty awesome, including Kyon giving an actual compliment, but considering the joke is now explained, all the humor is gone.

BakaHoushi said:
stuff

Okay, that explains a lot.
But I still don't get the "Future Diary" part.

The artist's comments also note that this image spawned a new pixiv tag, naga-koi-miku. 長古みく

I just grabbed volume 4 off my shelf. Koizumi himself calls it "Operation: Future Diary."

Why? Well, he's... Koizumi.

Edit: Actually, since they scripted the date, he might call it a "future diary" in the sense that a script is really a sort of a "diary" of future events. Which is stupid. But, again, Koizumi.

Kyon, stop being concerned with Koizumi's pseudo-date and be more concerned about your pseudo-date.

Grahf said:
Kyon, stop being concerned with Koizumi's pseudo-date and be more concerned about your pseudo-date.

Pseudouble date?