Virtually all researchers who believe that there is official interaction with alien species, agree that an official landing took place on 24 april 1964 at Holloman AFB in New Mexico. Most refer to this event as the Holloman landing. What the researchers do not agree on is whether this was the first such official landing or not. A majority, including the Serpo material, nowadays seems to think so. Others, including Val Valerian (who quotes a scientist claiming to have been present at the event), Bob Frissell and several sources in the Branton material, mention 20 February 1954 as the date of the first such landing, and Edwards AFB as its location (See Edwards Agreement). And a small minority mention dates in 1965 or even in 1972. Timothy Good on the other hand refers to probably two meetings at Holloman AFB: a first one took place in Feb. 1955, and may have been followed by another one in April 1956.
Personally, I tend to believe that the first 'official' landing took place in 1954 as it makes more sense in the whole sequence of events that had been unfolding up until that moment. This is also Val Valerian's point of view. He claims the agreement that was reached in 1954 was renewed at Holloman. The Holloman landing however does seem to be the one that led to the exchange of ambassadors. (See Plato Project, Serpo Project).