"I've got a musical message to deliver to 'Arry Potter in person."
-- One of Lockhart's "Cupid" Dwarfs, "twanging his harp in a threatening sort of way."
Later in the afternoon of Valentine’s Day, one of Lockhart’s “card-carrying cupid” dwarfs accosts Harry as he and the other Gryffindor second years head for Charms. The dwarf tackles Harry, sits on his ankles, and sings a Valentine’s message to him:
His eyes are as green as a fresh pickled toad,
His hair is as dark as a blackboard.
I wish he was mine, he’s really divine,
The hero who conquered the Dark Lord (CS13)
This happens just before Charms class, which judging by the the description is their last class of the day. Assuming a 4pm end of classes, this puts the time at around 3pm.
... late that afternoon as the Gryffindors were walking upstairs for Charms, one of the dwarfs caught up with Harry (CS13).