The dream of humans setting foot on Luna, or the moon as it was formerly called existed for over one hundred years prior to the July 21th, 1969 landing of the Apollo 11 mission. Even so, it was only after the "mountain of gifts" left by the Saran that it became economically viable to start a moon base.
In the century prior to the Saran's visit, most plans for a permanent moon base centered around a location near one of the poles, with the lunar south pole getting a considerable amount of attention. Even so, it would have taken a considerable amount of long term effort in order to succeed.
After the Sarans had left, and the United Earth was born, some members of the UE Congress pushed to start a colony on the moon. The choices were less limited now and the first sucessful permanent moon base was placed about a two kilometers from the site of the first manned landing on the moon.
That was done in 2091, with the base recieving the name Armstrong in honor of the first astronaut to set foot on the moon the following year. It was the first of the lunar bases to be reclassified as a city around that time.
Armstrong City reached 25 million people only three quarters of a century after it's founding, and at that time became the largest city in the Sol System. It's growth had already started slowing at that time and currently stands at 35 million people.