The question asks for 'real time' as to when people would have known the term 'sith'
DVK mentions the ANH scripts, but as far as I can tell these were not (officially) available until around the time of the special edition releases in 1997.
However, the official novelisation Star_Wars: From the Adventures of Luke Skywalker published November 12, 1976, does mention the Sith. (and notably Palpatine's name for the first time)
The novel also refers to Darth Vader as a Sith Lord. Although he was referenced as such in various merchandising tie-ins at the time of the original film, he is not referred to as a Sith Lord in the movie. In fact, the term Sith Lord is not mentioned in the films until 1999's Star Wars: Episode I – The Phantom Menace
I've looked through the star wars vault but the earliest mention I could see of Sith was on a lolly wrapper from 1980.
So I think the earliest 'canon' mention was in the novelisation in November 1976, but there would have been some unknown PR or merchandising that mentioned it before then possibly before the original film was even released.