Acting Headmaster/Headmistress
When the current Head is indisposed or absent, an Acting Headmaster or Headmistress is appointed. Typically this would be the Assistant Head, but this is not always the case. When Dumbledore was forced to leave Hogwarts after taking the blame for the student organization "Dumbledore's Army," the Acting Headmistress was not McGonagall, who is the Assistant Head, but Dolores Umbridge. Her position of High Inquisitor apparently outranked the Asssistant position. (OP)

antechamber
An antechamber is a small side room connected to a larger one. There are several small antechambers mentioned. One is connected to the Entrance Hall,  across from the Great Hall. In this room the new first year students waited nervously before being sorted into their houses at the Welcoming Feast. Another antechamber mentioned is off the Great Hall beyond the staff table. Here hang portraits of the Fat Lady's friend Vi and of a wizard with a walrus mustache. After being chosen as champions by the Goblet of Fire, Fleur, Krum, Diggory, and Harry met in this room with the officials of the Tournament.

April
Easter holidays are usually in April, giving the students two weeks off. This marks the end of the spring term and the start of the summer one. The weather can be quite rainy in April, but the temperatures are rising and spring is in the air. 

Important Events that Happened in April:
1992 - Norbert the Norwegian Ridgeback hatches
1994 - Gryffindor finally wins the Quidditch Cup after seven years of Slytherin victories
1996 - Dumbledore is replaced as Head by Umbridge. The Weasley twins set off vast numbers of enchanted fireworks against her.

armchairs
The Gryffindor common room is filled with comfortable, squashy armchairs. Harry, Ron, and Hermione have their favorite ones, right near the fire. 

armor
Suits of armor are all over the place in Hogwarts. Some of them are animated to some extent and they will turn their helmets to "watch" someone passing by. During the Christmas season, the suits of armor are enchanted to sing Christmas carols, at which time Peeves enjoys hiding inside and adding rude lyrics of his own to the songs. He has also been known to shut Mrs. Norris up in suits of armor.

Assistant Headmaster/Headmistress
While we do not know if this is typical of Heads of the school, we do know that Dumbledore appointed an Assistant Headmistress to help him manage Hogwarts and to take charge when he was away. McGonagall filled that role and took over when Dumbledore was replaced during the 1992-1993 school year and again after Dumbledore's death at the end of the 1996-1997 school year.

Astronomy
Subject taught at Hogwarts by Professor Sinistra. Classes take place at midnight. The first years had their Astronomy classes on Wednesday. [more...]

Astronomy Tower
The tallest tower of Hogwarts castle is the Astronomy Tower. It is located near the front door of the castle (since Harry could see the front door open and people walking out toward Hagrid's hut during his Astronomy OWL. Students are not allowed up in that tower except for classes. Harry and Hermione got in trouble for being in that tower when they had gone up to give Norbert to some of Charlie's friends for a trip to Romania.

artwork
Various types of artwork are displayed in the halls of Hogwarts, including paintings and sculpture. Many if not all of these are magical and are animated in some way. Apparently there are no art classes at Hogwarts, although Dean Thomas is known for being a good artist.

August
Rather boring month at Hogwarts. No teachers are present. Filch alone wanders the halls, cleaning up from the previous school year and getting things ready for the next. During August, Dumbledore selects the new Prefects and Head Boy and Girl. He also hires new teachers if any are needed. McGonagall oversees the sending out of letters to the students after Dumbledore has finalized these things (if this hasn't already been done in July).