This will be another time skip, sorry. I'll be putting this after later chapters, but I wanted to get it up in time for Christmas, so, enjoy!

Sherlock;
Saying Christmas was Sherlock's favorite holiday was a total lie. So was saying that he had no problem with you decorating him and John's flat to match the winter wonderland outside. But, you did it anyways... With John's help, of course. Sherlock came home to a brilliantly decorated, glistening frocked tree, surrounded by meticulously wrapped presents, fairy lights and garland, the last two, of which, sprung happily around the flat. In his confused state, Sherlock barely noticed you and John whirring around the kitchen, working to get all the baking done. Yes, many people described you as a machine most of the time, by Christmas was like that all just melted away in the starry lights and egg nog.
"Sherlock's parents will be here in about two hours." John commented as you nodded, letting forth an agitated grunt.
"I am in no way, shape, or form, ready for this." You rolled your eyes, continuing to delicately ice each cookie. Forcing himself from his shock before it killed him, he began moving towards your voice, finding the kitchen worse than his lounge. More garland and fairy lights, yes, but also wreaths every two centimeters, and a mistletoe hanging in the doorway. You caught a glimpse of his frazzled state, John already taking the time to laugh at it, and instantly wanted to spur out apologizes and explanations.
"Your parents are on their way." Was all that came out. With a few strings of curses and a LOT of coaxing, you convinced the detective that festivities and family weren't all too bad. His parents were sure to get a picture of you two under the mistletoe, though, and suddenly, Sherlock readily agreed with your 'theory'.

John
You two were Christmas-decorating machines! He went around your flat with emerald garland, you following with ruby tinsel, and then a pattern of wreaths, and stockings, and snowflakes, and mistletoe. The tree ended up looking gorgeous, it's long pine needles had made it difficult, but it turned out amazing. Most of the ornaments were simple balls, and the occasional reindeer or snowflake, yet six were little Christmas-themed doctors, or doctoring items. You had priorly done the exterior, it's patterned lights matching the interior's garland, lining the roofing to your home, and your doorway. The best touch was what John did with he lighting. He inserted see-through coloured glass in place of the clear, giving your actual lights green and red tinting. The only thing left to do was decorate the cookies!

Mycroft;
When you entered your flat with the decorations in tow, he looked ready to collapse. He really didn't like where this was heading...
"Is it really necessary to 'decorate' your flat for a ridiculous holiday for the birth of a man, who wasn't even born on the celebrated day?" He complained while helping you wrap garland around the tree. You inwardly sighed. You knew Mycroft would have his reservations, but Christmas was one of your favorite times of the year, and you were psyched to see your family for the holidays.
"Yes My, it is. It most definitely is." You said the line for what felt like the thousandth time. He groaned a tad bit, before returning to his usual neutrality, which was definitely fine by you. By the time you got to the star, though, you swore Mycroft was beginning to enjoy himself. His lips curved slightly up into a small smile as he watched you put the glittering gold star above the mainly silver and gold tree, with hints and flashes of a smooth blush colour.
"Look who's smiling! Enjoying yourself, Mycroft?" He was caught a bit off guard, but his soft smile returned as you wrapped your arms around his neck.
"Perhaps I just like watching you decorate. It's much too much leg work for me." He insisted, pressing his lips to yours despite himself.

Moriarty;
Jim was, how do we say, excited for your first Christmas together. For him, the year usually consisted of listening to Sebastian complain he was WAY too into this 'holiday crap' for someone who claims hating ordinary things, and after seeing what he had done to your home, you readily agreed. You walked in, carrying your groceries and other necessities for hygiene and other crap, and nearly dropped it all once you walked through your door.
"Jim, what the hell?" You looked around your flat, eyes following the black and gold garlands to the tree, which continued the pattern, having gold, black, and crimson ornaments. Weirdest part? There wasn't a star. There was a skull. An actual skull! Upon a closer look at the decorations, you noted that there were a few blades and guns along with the usual balls and glass crap.
"...I decorated?" He tried, already knowing he was in the doghouse for this one.
"Well, you better be the one cleaning this crap up, then. I'm sure as hell not doing it." You smirked at him, and he just smiled and came to you, wrapping his arms around your waist.
"But of course! As long as I get a kiss."