A bunch of Overwatch hero interactions and sound effects have turned up on Reddit, one of which hints rather strongly that a holiday-themed event is on the way. That seasonal Overwatch festivities are likely to happen is probably one of the least-surprising things you'll hear all day, but Blizzard hasn't announced anything yet and so for now, speculation is all we've got.
Official or not, the Christmas theme song perfectly fits the part. Give it a listen below—you can almost hear the bells jingling in the background, while icicles twinkle in the snow-lit night. The rest of the audio clips are somewhat less overtly festive, but still fun in their own right: Tracer expresses a wish that Sombra would "just disappear forever," while Reaper warns Sombra to "stick to the plan," and Reinhardt finally beats Zarya at arm-wrestling. (She heard it was his birthday, though, so she went easy on him.) There's also a reference to "entering the Arena," along with seemingly-related proclamations of "Challenger" and "Gladiator," made by Winston's AI Athena, perhaps suggesting that a new mode is in the offing.
I don't expect that Blizzard will confirm that these clips are authentic (I'll let you know if it does), but short of Mike Morhaime tweeting a double-thumbs-up selfie, this is as close to a sure thing as we're going to get until an official announcement is made. Blizzard said in its third-quarter financial results that the Summer Games event "drove record engagement and participation in in-game customization items," while the Halloween Terror event "drove even higher engagement and participation." And Morhaime said in a follow-up call, via Gamerant, that players "should expect to see more events" like those in the future. A future that I would bet we'll be hearing about any day now.