In an event called "Breaking the Bar", the Half-Life community decided to rally together and get as many concurrent players together as possible. The rules were simple: starting at 3pm UTC, everyone who wanted to take part simply had to run (and ideally play) Half-Life 2, while prominent streamers would be playing the game to raise attention. The reasoning is maybe a little more ambiguous, but sometimes you simply have to rally behind something—and they did it, hitting a peak of 16,101 concurrent players at exactly 3.30pm, with an additional 8,630 viewers on Twitch.
An interesting detail here is that Half-Life 2 is, well, old. Released in 2004, it predates many of Steam's features, including the ability to track data like time spent in a game and concurrent players. The goal they had to beat was 6,882 concurrent players—which they more than doubled—but we can't actually know how many people played Half-Life 2 in 2004. Were there ever more than 7000 concurrent players before now? Add it to the life stats you get to be eternally curious about.
It's perhaps all the more impressive, then, that a community would get behind a game in such numbers sixteen years after its release. It's not the only such community effort, either: Half-Life 2: Remastered Collection was recently listed on SteamDB, and is an in-process mod from the same group as did the Half Life 2: Update mod.