Van Halen’s debut album was released on Feb 10, 1978 and was certified 1x platinum on Oct 10, 1978. They had a party at The Body Shop on Sunset Blvd.
They did a quick tour run in Europe from Oct 9 – Oct 22 with Black Sabbath again. And there’s an article in the paper on Oct 28 about this party. So it’s not clear the exact date, estimating 10/24 here.