The main story of Stranger Things begins in 1983.
Season 1 opens in November 1983, when Will Byers disappears while returning home from a game of Dungeons & Dragons with Mike, Dustin and Lucas. His disappearance leads to the discovery of Eleven and the existence of the Upside Down.
The year is important because the series uses real 1980s culture, technology and politics as part of its atmosphere. Later seasons move forward chronologically: season 2 takes place in 1984, season 3 in 1985 and season 4 mainly in 1986. Flashbacks show earlier events at Hawkins National Laboratory, but the central narrative starts in 1983.