Welcome to this story! In my mind, it happens halfway in Season 3. Kagami and Luka exist but the season 3 finale hasn't happened.
The story starts slow but I wanted to give Adrien/Chat Noir and Marinette enough time to get to know each other. It is not too Christmassy, the focus is on Marichat/Adrienette falling for each other. Hope you enjoy reading, the story is now completely planned out and should be finished by the end of April. If you are really impatient, you can skip to Chapter 5. That's where the pace picks up (I think).

PS: For the cover, I adjusted a photo from goodfreephotos.

Marinette loved the weeks before Christmas. Her room basically turned into a gift workshop, as she stocked up on fabric and wrapping paper, plastered her wall with post-its to keep track of ideas and slowly built a pile of home-made gifts.
Tonight, she was working on a bandana for Alya. It was late, the house was quiet but for the regular sound of her sewing machine and Marinette was completely absorbed by her work, when a gentle knock at the window made her look up. There was a dark figure outside, and a year ago, Marinette would have jumped to the other endof the room with a shriek. Not anymore though - she had learned to control her fear (well, most of the time, unless Adrien was talking to her) and she instinctively recognized Chat Noir as he dangled head down in front of her window.

Adrien didn't know what he was doing at Marinette's place. After a particularly tiring reception for a fashion show, he had needed to cool off. No better way to escape than to speed across the roofs of Paris, letting the cold air brush away worries about whether he had met his father's expectations today. Adrien loved that on these tours, he would sometimes catch a glimpse of people's lives behind the well-lit windows. He especially liked to see families together, seeing them briefly at the dinner table or parents reading bedside stories to their kids.
Tonight he had been so late that many windows were already dark. So when he had passed Marinette's place, he had been surprised to see that there was still light on at the top floor. He had watched her working at her desk for a moment from the roof across the street, but then he felt creepy and in a spontaneous move, decided to pay her a visit instead.

"Chat noir!", Marinette said in a carefully friendly tone as she opened the window to let in a cold breeze and the grinning cat boy who rolled unto his feet in front of her. "Is everything alright?" She peeked at her phone, but there was no Akuma alarm.
"Of course, everything is –" he stopped mid sentence, taking in the scene in front of him. "Wow", he said.
Marinette followed his gaze and shrugged. "Yeah, it gets a little crazy here before Christmas."
She closed the window to keep out the cold. When he did not speak, she explained the obvious: "I am working on presents for my friends and family."
Chat Noir nodded, looking at the pile of gifts with fascination, before his gaze fell back onto her.
"That is so nice of you, Marinette", he said politely. "I'm sorry, I didn't mean to interrupt your work. I guess it just looked so cozy and inviting in here.."
He trailed off again and scratched his neck in a nervous gesture.
Huh? Marinette looked at the sheepish figure in front of her in confusion. He was being awefully – polite? And that helpless smile he was giving her was almost.. cute? No cheeky puns, either. And he didn't help himself to one of the delicious Christmas cookies on her desk! The poor cat seemed to be really off his game tonight. He looked tired, too. Maybe there was something off in his private life. Over the past months, she had pieced together that Chat Noir's family life was not all rosy. He was feeling lonely and whatever was going on, it was putting a strain on him.
She decided to be nice to him. It was Christmas after all. Maybe he needed someone to talk.

Marinette didn't ask what he wanted from her. Instead, she offered him a plate of Christmas cookies.
Adrien was so grateful for her kindness, he could have hugged her. Seriously, that girl had to be the best friend anyone could wish for.
But he didn't hug her, of course. Instead, he selected a chocolate covered, crunchy little masterpiece from her plate.
It was so delicious, he was practically purring when he decided to take a second one.
Marinette chuckled. "So", she said, "Hot chocolate?"
Apparently his expression told her enough, because she just nodded and left him in the room with a relaxed "Don't touch the sewing machine!"

Adrien would have loved to come along and learn how to make hot chocolate. The idea of sneaking into the dark kitchen together was.. exciting. But he didn't, of course. It was not a good idea to risk waking up Marinette's parents. If they found them together, she would have to explain what he was doing there late at night. And the last time her parents had caught them together had ended in a very awkward way. Not to mention with Marinette's father being accumatized. He blushed when he thought about it.
He picked up a magazine from Marinette's desk (100 DIY ideas for Christmas) and sat down on the canapé to read. She had trusted him by leaving him in her room alone, so he would obviously not sneak around. But he couldn't help noticing the pile of colourful ribbons on her desk. He was curious. What was she making? And would she make him, Adrien, a present, too?