From the Beholder periodical level entry:
Although there were no publication dates printed on the early editions of The Beholder, the first issues can be placed as being published in 1979 from references in other contemporary fanzines.
In Demonsblood issue 1, which was published in January or February 1979, there is an article by Michael Stoner. At the end of the article Brian Dolton, editor of Demonsblood, added the following Stop Press:
"Since I received and retyped this article I have heard from Michael that he is starting his own 'zine, of Underworld Oracle style. Called "The Beholder", it will be 28pp, 40p an issue, appearing monthly with the first due in April."
Mike and Guy were very reliable with the publication schedule of The Beholder, and although it can't be definitively stated it seems very likely that The Beholder Issue 1 was published in April 1979, with other issues appearing at monthly intervals thereafter.