Each common room at Hogwarts has a noticeboard. The noticeboards are used by the Hogwarts staff and students to post the dates of Hogsmeade visits, Quidditch training schedules, Apparition lessons, as well as lost and found notices etc. Umbridge uses the noticeboards to post her Educational Decrees. The Weasley twins use the noticeboard to advertise their Wheeze products and to request testers.
"A large sign had been affixed to the Gryffindor noticeboard, so large it covered everything else on it - the lists of second-hand spellbooks for sale, the regular reminders of school rules from Argus Filch, the Quidditch team training timetable, the offers to barter certain Chocolate Frog Cards for others, the Weasleys' latest advertisement for testers, the dates of the Hogsmeade weekends and the lost and found notices" (HBP17).
"To general indignation, a sign went up on all common-room noticeboards that the next trip into Hogsmeade had been cancelled. Ron was furious. 'It was on my birthday!' he said. 'I was looking forward to that!'" (HBP18)