It's the more sane of the two 2009 internet Haruhi Suzumiya spin-off comedy series. They were released on YouTube together from February to May, both using the parent show's voice cast. They're both random absurdist surrealism, but at least this one's comprehensible. It's quite fun.
It's basically the characters and situations from Haruhi Suzumiya, but drawn in cute cartoonish style ("chibi") on almost no budget. It looks like garbage. It's an internet anime. So be it. Mind you, later on they start experimenting with occasional inserts in more realistic styles, all the way up to art that's indistinguishable from the parent show's. That was quite impressive. Admittedly the characters always transform back to their chibified selves for more silly comedy, but it still adds more than you'd think.
Is it a pisstake? Sort of, but not quite. A lot of these episodes could be regular Haruhi Suzumiya adventures, except played for laughs and delivered at a manic pace. The cast are recognisably themselves. You don't need to update anything to play the Haruhi-Kyon relationship for comedy, for instance. Haruhi abuses Kyon outrageously. No change there. It's all too manic and crazed to be read as "real" in-universe adventures, but it's still clearly them, doing exaggerated versions of the same kinds of things. The nearest I can get to acting out of character is the callousness about each other's potential deaths. (This is funny and I'm not objecting at all, but it is out of character.)
That said, though, many of the events here did indeed occur in the books. The Valentine's Day chocolate cook-off is mentioned at the end of book 2, Sigh, for instance. There are also lots of references to events in the parent series, making this effectively a big sequel.
Secondary characters crop up more than they did in the main show, I think. Agency members Arakawa and Mori (the butler and maid in Remote Island Syndrome) get promoted to semi-regulars.
The biggest format change involves Yuki. She still has the same almost emotionless personality, but she's swapped books for pornographic lesbian computer games and she ends up being the head of a very odd household. Its other two members are (a) a resurrected killer in harmlessly cute, tiny form, and (b) a sentient balloon animal called Kimidori-san.
The show does Christmas (ep.19), Halloween (ep.13) and Valentine's Day (ep.11), by the way.
It takes a few episodes to get going, I think. Realism's arguably not much further out of the window than it was in the parent series. They're just being more manic about it. It's often bonkers rather than funny, admittedly, but then the ever-mounting surrealism might reach a pitch that makes you laugh, even though you're not sure why. I like its bouncy music. I admire its absurdity, which can attain brilliance. As a comedy, it's hit-and-miss but often good. As an exercise in deranged high-octane fun, it's well worth a look.