Anno 1602—or 1602 AD, as it was known in North America—was originally released in 1998, which in the context of videogames might as well be 1602. But it's an RTS/management game, which generally hold up to the years better than other genres, and even graphically it's not too bad: Dated but far from ugly. All of which is relevant right now, 20 years down the road, because from now until December 22, it's free for everyone.
To get it, you'll need to log in with your Ubisoft account—you do have a Ubisoft account, right?—at which point it will automatically be added to your Uplay library. You'll be given the option to either launch Uplay or download it if you don't already have it installed, or you can just go off and do something else at this point, confident in the knowledge that if you ever do have the urge to play Anno 1602, it will be waiting for you.
Anno 1602 was a multi-million-selling hit in Germany, where it was first released (it was developed in Austria) but reaction to it elsewhere was somewhat more mixed: PC Gamer UK scored it 81/100 in its review, for instance, while PC Gamer US gave it a less enthusiastic 57/100. (Interestingly, the actual reviews aren't terribly dissimilar: The US said it's "tough to call," the UK described it as "depending on your disposition," which really emphasizes the importance of reading reviews rather than just diving at the scores.)
But it was successful enough to kick off a long-running series of games that's still active: Anno 1800, a return to the series' historical roots, is set to come out on February 26, 2019.