Age 11
Severus was surprised when Lily and her father showed up at his house. Her cheeks flushed with cold, she had excitedly asked if he wanted to have Christmas with her family. Christmas spent with Petunia's scowl was better than Christmas spent with his parents' arguing, so after an approving nod from his mother, he said yes.
The Evans family wasn't rich by any means, but from just looking at their festively decorated house, Severus felt self-conscious about his bleak abode. He knew they had to be judging him and his mother for his ill-fitting clothes.
Mrs. Evans welcomed him in with a warm smile and the scent of gingerbread cookies, while Petunia stood behind her mother, glaring. Severus ignored the Muggle sister as Mrs. Evans ushered him into the dining room for a meal that was only slightly less delicious than the food made by Hogwarts' House-Elves. Afterwards, they exchanged presents, and it became clear to Severus that his invitation had been last-minute: He received a pair of socks and a man's sweater.
"Sorry," Lily whispered.
He wanted to tell her not to be sorry, wanted to say thank for the sweater that his parents didn't have the money to buy, wanted to say that this was the best Christmas he had ever had. But the words got stuck in his throat.
Age 12
Severus was relieved that his father hadn't come home yet when Lily and her dad knocked that next Christmas. Tobias Snape loved only the bottle, and since the old mill was closed on Christmas and he had the day off, he and his mates had gone down to their favorite bar to drink and gamble their wages away. When he had finally wasted all his money, he would stumble home and yell at his wife until he passed out.
This year, the Evans family had put more planning into him spending Christmas with them. Severus got socks and a sweater again, but this time they had been knit just for him. Both made from a serviceable wool, the sweater was dark green with a grey-and-white design around the neck, and the socks, clearly made by a less experienced knitter, were grey-and-green striped.
In fact, every member of the Evans family got a pair of somewhat lumpy socks that year—everyone except Lily, who was smiling with nervous anticipation when Severus tried on his socks, and who tackled him in a hug when he awkwardly murmured, "Thank you."
Age 13
Severus hurried to the Evans house as soon as his father left to go drinking.
He hated being desperate, but ever since Eileen had died, Tobias had been even more cruel, and Severus couldn't stand the thought of spending the entire winter break without his mother to shield him from his father's rages. He felt a bit cowardly, hiding like this, but self-preservation was in Slytherin's House values, not idiotic Gryffindor bravery.
With a bit of help from Lily's convincing and Mrs. Evans' tenderheartedness, the family took him in and let him spend Christmas break sleeping on their couch. For the first time—outside of Hogwarts—he was consistently fed and warm.
Age 14
Severus stayed at Hogwarts for Christmas. He had no reason to go home—his father had become even more volatile of late. And besides, Lily was also staying at Hogwarts, because her parents were travelling for some sort of family thing for the holidays, and Lily didn't want to stay with her maiden aunt, like Petunia was.
The holiday should have been relaxing, but Severus spent the entire time trying to placate Lily and his friends, torn between the kind-hearted Gryffindor and the scheming Slytherins. Lily didn't understand why he would want to spend any time with his Housemates, even offering to let him sleep in her dormitory. The Slytherins didn't understand why Severus even tolerated the Gryffindor bothering him. Usually, Severus could keep the two parts of his life separate, but without the distraction of classes and other students, it was difficult to explain any time he spent with the other friend group.
Age 15
Severus chose to spend Christmas at Hogwarts again, this time at the urging of his fellow Slytherins.
Mulciber and Avery, though talented with curses, were complete dunces when it came to potion-making. They needed him to brew something, an advanced Dark potion with a recipe that could only be found in the tomes of the Restricted Section of Hogwarts library, and ingredients that would have to be stolen from Professor Slughorn's private stores.
Luckily, Avery was decent at forgery, so he was able to create a signed note from Slughorn that allowed Severus to obtain the necessary book from the Restricted Section. Severus had to steal the ingredients himself; neither Mulciber nor Avery were inconspicuous and slight enough to sneak in and out of Slughorn's stores.
The potion itself was one of the most complicated Severus had ever attempted, and he welcomed the challenge. The brew demanded a great deal of attention and coddling, but Slughorn never questioned why Severus was so often missing—the spidery boy frequently disappeared for long periods of time anyway.
Age 16
Severus sat alone at Spinner's End, waiting for the Christmas invitation from Lily that would never come. Last summer, his father had finally drank himself to death, leaving Severus the house with its leaking roof and thin walls that did nothing to stop the cold. He didn't have any money to pay a repairman or buy warmer clothes, and wasn't old enough to legally do magic, so he could do nothing to better his current situation.
He was left to shiver by himself, with only dreams of Death Eater glory to distract him from his misery.