This is all very confusing. In New York City in the mid 70s there were at least three rival Star Trek conventions with Star Trek Convention being the oldest. The Star Trek Lives! committee split in 1974, with its chairman, Al Schuster, leaving to create a rival convention.
He kept the name (International Star Trek Convention) from the previous conventions while the committee started using Star Trek Convention for theirs. Many in fandom distinguished the new Al Schuster-run conventions from the old committee-run conventions by calling the Schuster-run series "Al Schuster's International Star Trek Convention", and we will adopt that here.
Al Schuster's 4th Annual International Star Trek Convention was scheduled a month before Star Trek Lives! (the Star Trek Convention) so that in 1976 things came to a head with three New York Star Trek conventions in a two-month period. Al Schuster's 4th Annual International Star Trek Convention was first, being held on January 22-25 at the New York Hilton, the Star Trek Convention 1976 was on February 12-16 at the Commodore Hotel (which had a membership limit of 6000), and a commercial Star Trek convention two weeks later.
The Schuster-led convention attracted between 20,000 and 50,000 people according to the New York Times with many ticket holders being turned away when the facilities were completely swamped. (Its committee seems to have been entirely unprepared and quite disorganized.) The New York Attorney General investigated and both fan-run conventions ended. See also Star Trek Fiasco.
This had a fortunate side-effect in that it probably helped to kill the SF Expo, an convention planned for June of that year at the New York Hilton which was an attempt to create a regular SF convention on the scale of the Star Trek conventions.
See Star Trek Conventions for more.