What date did the planning stages of GOES-1 begin?

I combed through all Wikipedia entries on GOES-1 and was unable to find when the planning phases of GOES-1 started. I see how it all says it was launched in 1975 but even after ashamedly going to AI to see if it could catch anything I missed, well that didn't end well and left more confused why the program planning phases dates at a bare minimum weren't documented anywhere online.