How do you think the Soviets went undetected in Hawkins so long? We know they started to plan to move their operations to Hawkins at least in the summer of '84 (or a bit later), so if they started that before December, they would've had to be extremely cautious so Hawkins Lab didn't pick up on what they were doing. I don't know how long it takes to build a mall+base, even with a corrupt mayor on your side, so do you think it's possible they were there earlier than that, as Murray claimed in s2?
I’m not sure exactly when their plan started, but I would honestly guess that it was before the events of season 1, or right around then. I mean just look at the difference in technology and understanding of the Upside Down. A year after the gate had opened in America, American scientists were still scratching their heads and didn’t understand anything except that it could be killed with fire. Seven months after the gate had opened in America, the Russians already had a prototype machine to open the gate themselves. The Russians clearly had a much deeper understanding of the Upside Down than the Americans, and the Americans had the advantage of having access to it. So honestly, I wouldn’t be surprised if the Russians actually knew about the Upside Down before the events of season 1, but then struggled with figuring out how to open it for so long. If not before s1, then I think they learned right around the same time as the events of season 1. Maybe there was a mole in the American government, maybe the the Russians were able to track the change in energy from El opening the gate, maybe Brenner ran off to them, idk, but I think November-December 1983 is the absolute latest their plan to open the gate could have started, and it’s very likely it was even earlier.
As for actually moving in to Hawkins, I bet that happened right around the end of 1984. In November 1984, the lab was in shambles and about to be shut down, which means they wouldn’t be monitoring things as closely. Mayor Kline would be desperate for some good press, and would definitely have jumped at the opportunity for a new mall in town. And the general public’s attention would be diverted to the alleged chemical leak. It would have been the perfect time for the Russians to move in. So yeah I think late 1984 is when they started coming.
However, there is an interesting question I think where even though they probably didn’t come to Hawkins until after the events of s2, were their plans to come to Hawkins existing pre-s2? Alexei said that they decided on Hawkins because the gate had been opened there before, which subtly implies that the Russians didn’t decide to come to Hawkins until after Eleven closed the gate, but I wouldn’t be surprised if they were already considering it beforehand anyway. Maybe even before the events of s2 the Russians started making some initial plans (like some of the farmland purchases) just in case they decided going to Hawkins was their next course of action