The singing grew louder as they approached the church. It made Harry’s throat constrict. It reminded him so forcefully of Hogwarts, of Peeves bellowing rude versions of carols from inside suits of armor, of the Great Hall’s twelve Christmas trees, of Dumbledore wearing a bonnet he had won in a cracker, of Ron in a hand-knitted sweater ... -- Harry in Godric's Hollow (DH16)
Each year the teachers at Hogwarts decorate twelve Christmas trees for the Great Hall.
First Year: Flitwick and McGonagall finished decorating twelve trees for the Great Hall. Some trees were had tiny icicles, while others had "hundreds of candles" (PS12). When the hallway outside Potions class was blocked by Hagrid and one of the trees, Draco taunted Ron, saying he was a future gamekeeper whose house was less fancy than Hagrid's Hut. Ron was about to hit Draco, so Snape took ten points from Gryffindor even though Hagrid told him about the insult (PS12).
Second Year: The twelve trees were "frost covered" as enchanted snow fell from a ceiling festooned with holly and mistletoe (CS12).
Third Year: Madam Rosmerta had a "handsome" Christmas tree near the fireplace at the Three Broomsticks in Hogsmeade. Hermione floated it closer to their table so they could eavesdrop on their teachers nearby (PA10) . The twelve trees of Hogwarts were hung with golden stars (PA11).
Fourth Year: In honor of Durmstrang and Beauxbatons students visiting Hogwarts for the Triwizard Tournament, the trees were decorated with "everything from luminous holly berries to real, hooting, golden owls" (GF22).
Fifth Year: At Grimmauld Place, a large Christmas tree was placed in front of the Black Family Tapestry and Mundungus decorated it with live fairies (OP23). While visiting Arthur Weasley at St. Mungo's hospital, Harry saw "shining white" trees in the reception hall covered in snow and icicles, with golden stars on top (OP23).
Sixth Year: Hagrid delivered the usual twelve trees at Hogwarts, but no special ornaments were seen (HBP17). At the Burrow, Ginny decorated with paper chains and Fred used a living gnome as a tree-topper: "Stupefied, painted gold, stuffed into a miniature tutu and with small wings glued to its back, it glowered down at them all, the ugliest angel Harry had ever seen, with a large bald head like a potato and rather hairy feet" (HBP16).
Seventh Year: Harry and Hermione visited Godric's Hollow. They saw Christmas trees shining in the windows of the houses (DH16, DH17), and one "windblown" tree in the town square near the monument to Harry's parents (DH16). The scene made Harry sentimental as he remembered happy years of Christmas past at Hogwarts.