Possible backstory to the Christmas jumper:
The jumper was an early Christmas gift from Sherlock. Sherlock worried it was a bit much and that John would be offended by this gentle poke at his fondness for jumpers. But John was delighted; ‘Oh, this is very nice’, he said - somewhat hoarsely to Sherlock’s ears - lifting it slowly from the box. John insisted on wearing it for their drinks party, and Sherlock was puzzled when before the guests arrived he caught John gazing wistfully at his reflection in the mirror over the fireplace. He didn’t know that when John was a lad he and his gran had a tradition of shopping for his Christmas jumper early every December. John gladly deferred to his gran’s flamboyant taste in colour and design in making the purchase, which then became the center of interest and admiration at the Christmas family gatherings - until the year he turned 14. That year his first girlfriend had mocked his gran’s choice and the following year he’d told his gran he was too old to wear a special Christmas jumper. She’d said nothing but he’d seen the sadness in her eyes. She died before he grew up enough to revive the tradition, so he’d never worn an outrageous Christmas jumper again. But this year he had one from Sherlock, and if he saw disdain in Jeanette’s eyes when she looked him up and down at the door, well then, too bad for her.