March 31, 1949 was the kick-off day. according to this site:
However Wikipedia says:
Unwilling to accept and license Columbia's system, in February 1949 RCA
Victor, in cooperation of its parent, the Radio Corporation of America,
released the first 45 rpm single, 7 inches in diameter with a large center
hole. The 45 rpm player included a changing mechanism that allowed multiple
disks to be stacked, much as a conventional changer handled 78s. The short
playing time of a single 45 rpm side meant that long works, such as
symphonies, had to be released on multiple 45s instead of a single LP, but
RCA claimed that the new high-speed changer rendered side breaks so brief
as to be inaudible or inconsequential. Early 45 rpm records were made from
either vinyl or polystyrene <http://en.wikipedia.org/wiki/Polystyrene>.
<http://en.wikipedia.org/wiki/Gramophone_record#cite_note-35> They had a
playing time of eight minutes.
So On March 31, 1949, RCA Victor released "Texarkana Baby" b/w "Bouquet of
Roses" by Eddy Arnold.