Harry slips into an unused classroom to avoid being caught out of bed by Filch and Snape. Inside the classroom is a tall, ornate mirror. When Harry looks in it, he sees his parents and generations of his family. Harry remains transfixed by what he sees until a noise startles him back into awareness. He hurries off to his dormitory, but not before promising the people in the mirror that he will return.
Christmas in 1991 was on a Wednesday.
This silent, peaceful night Harry spends with his parents is Christmas night. Perhaps Rowling is comparing the Potters to Christianity's Holy Family (Jesus, Mary, and Joseph) by alluding to Christ's nativity. -BB
When Harry first sees his relatives in the Mirror of Erised, he thinks they might be otherwise invisible people standing in the room with him. Later on, Dumbledore reveals that he had made himself invisible and observed Harry become fascinated with the Mirror. -BB
Many of the people in the Mirror have Harry's (and Lily's) eyes. This means that he is not only seeing his father's magical family, but also his mother's presumably all Muggle family. Pottermore has provided more information on the Potter lineage, but little is known about the Evans line. The presence of Lily's blood relatives (sans Petunia and Dudley) suggests that Petunia and her family may very much be the black sheep of the Muggle side of Harry's family. -BB