"We think the reason Umbridge doesn't want us trained in Defense Against the Dark Arts is that she's got some... some mad idea that Dumbledore could use students in the school as a kind of private army. She thinks he'd mobilize us against the Ministry."
-- Hermione Granger (OP16)
Hermione invites interested students to the Hog’s Head, and twenty-eight people show up.
In the dirty, seedy Hog’s Head pub, where many of the more typical patrons keep their faces covered, students gather who are interested in going around Umbridge and learning more than just theory of Defense Against the Dark Arts. Zacharias Smith raises the whole issue of Harry witnessing Cedric’s death and Voldemort’s return, Luna and Hermione disagree about whether or not heliopaths exist, Ernie MacMillan feels the need to make a speech, the importance of Quidditch practices presents an obstacle for scheduling meetings, and no meeting place can be decided on. But everyone eventually agrees to sign up on a piece of parchment and to keep the group a secret. Harry stresses that, while he knows a lot and is willing to teach them, there was a great deal of luck and help from others involved in the things his classmates credit him with.
After the meeting ends, Cho lingers until her friend, Marietta, gets her to leave. Ron is upset when he learns that Ginny is dating Michael Corner.
This event occurs on the first weekend in October (OP16), which would make this October 5.
After everyone signs their name, there is a feeling as though a contract had just been signed. In fact, of course, they have, thanks to Hermione having bewitched the parchment.
Note that every Gryffindor in Harry's year, other than Seamus, has signed up. JKR is setting Seamus up as a red herring for the candidate of betrayer; after months of meetings when all four of his dormmates were out until curfew, there's no way he couldn't have known something was going on, particularly since Dean seems to be his best mate.
Note the selection of people: the Gryffindor team and their friends, Ginny's boyfriend and his friends, and the non-Slytherin fifth year prefects and their friends. (Parvati qualifies on two counts, as Hermione's dormmate and Padma's sister, of course.) I'd guess that Colin and Dennis are in because Colin and Ginny are classmates. We know that JKR shifted Corner and his friends from Hufflepuff to Ravenclaw while writing OP (we know this from interviews showing some of her working notes). Personally, I suspect that she did this to try to keep the DA's membership more or less balanced between the three non-Slytherin Houses.
Since Dennis Creevey is only a second-year, having been sorted into Gryffindor the previous September (GF12), it seems to be an inconsistency that he's in Hogsmeade at all, since in previous years the school rule was that only third-years and older students could go to Hogsmeade.
Among the patrons of the Hog's Head is a man drinking some dangerous-looking liquid and whose entire head is covered in filthy bandages. This is reminiscent of the beginning of H. G. Wells' The Invisible Man, in which the reader is introduced to the bandage-covered title character as a mysterious and unfriendly patron of an inn. Perhaps the bandaged man in the Hog's Head is invisible. -BB
It is amusing to reread this scene with the knowledge that the cranky barman is Albus Dumbledore's younger brother, Aberforth. -BB
In the Hog's Head, Harry spots a veiled witch and worries that it could be Umbridge. Well, it's not Umbridge - it's not even a witch - but Harry IS being spied on by that person. Sirius later reveals that the individual under the veil was Mundungus Fletcher, disguised and keeping an eye on Harry. -BB
Hermione hesitates before asking everyone to sign the parchment. She seems to have gotten past her nerves at this point, so it seems safe to assume her hesitation has to with being about to have people sign their names on a piece of parchment without telling them that it's jinxed. -BB