June 21st, 1948:
https://www.theguardian.com/music/2023/jun/21/columbia-records-introduce-first-vinyl-lp-1948https://blogs.loc.gov/now-see-hear/2019/04/inside-the-archival-box-the-first-long-playing-disc/https://www.wired.com/2010/06/0621first-lp-released/
As Wired notes, though, RCA had actually tried to market an LP as far back as 1931:
https://www.theguardian.com/music/2020/oct/28/long-playing-record-innovation-1931
https://www.theguardian.com/music/2023/jun/21/columbia-records-introduce-first-vinyl-lp-1948https://blogs.loc.gov/now-see-hear/2019/04/inside-the-archival-box-the-first-long-playing-disc/https://www.wired.com/2010/06/0621first-lp-released/
As Wired notes, though, RCA had actually tried to market an LP as far back as 1931:
https://www.theguardian.com/music/2020/oct/28/long-playing-record-innovation-1931