April 1st, 1963 - The Soap opera "General Hospital" premiered on television.

http://images.movie-source.org/thumbs/10082_General_Hospital_1963.jpg

So did another hospital-set soap, The Doctors, which flatlined at the end of 1982. I remember a quote that original cast member John Beradino (Dr. Steve Hardy) said when the show wasn't getting good ratings. Referring to the show starting on April 1st, at every renewal he said, "Well, we managed to fool them for another year." :D

Who is GH's current longest running cast member???

I think that would be Leslie Charleson (Dr. Monica Quartermaine), who joined GH in 1977. Jacklyn Zeman (Bobbie Spencer) joined later that same year.