set in the current season
(Castle)
He sees the mittens in her office when he sneaks in to leave her a surprise. It both warms his heart and shocks him a little, because he honestly never expected her to ever wear them.
He'd found them at a farmer's market last year, had bought them on a whim. Something cute to stuff Kate's stocking, and he'd actually gotten matching pairs for himself, Alexis, and Mother.
They're actually fingerless gloves that convert to mittens, made from brown wool, and made to look like reindeer. A bright red button holds back the mitten top, which is decorated with felt antlers.
They're completely adorable, and nothing he ever expected his wife to wear out in public, let alone proudly display on her desk.
The tops are folded back, and it gives him an idea. He'd bought tickets for The Nutcracker, a way for them to spend time together that had a very low probability of them actually being recognized together. His original plan was to leave her ticket somewhere on her desk. Maybe in her notepad, or in her inbox.
Instead, he tucks the ticket into the mitten with a folded up note. I'll wear mine if you wear yours...
(Two nights later, they leave the theater together, mittened hands entwined.)