Stardew Valley update 1.3 is the big one, ie, the one that will introduce (official) multiplayer functionality. And according to creator 'ConcernedApe' the functionality has been completed, thanks to the work of Chucklefish's Tom Coxon, who has "done all the network coding to turn Stardew Valley into a multiplayer game."
As for when it's coming, that'll be after some rigorous QA testing. "Work on the new features is done, and we’ve sent all the new text off to be translated into the different languages," ConcernedApe wrote.
"Meanwhile, we are working on bug fixes. Once we’ve received the translated text and integrated it back into the game, we’ll begin a serious QA phase involving thorough testing of all new features in both multiplayer and single player contexts. When that is finished, the update should be ready for a public beta. I am still shooting for a spring release for the beta."
Spring, for the sizeable portion of the population not based in the northern hemisphere, including Australia, means an autumn release. More info will come closer to the beta, but in the meantime, that image embedded above demonstrates how the new signs will look. You can read the full post here.