what planet/moon did the ceremony at the end

I always assumed it was the planet where all the rebel ships launched from in the finale. Can't remember the name.

It was an earth-like moon covered in an alien jungle orbiting the orange gas giant of Yavin. Since it was the fourth moon orbiting Yavin, it was called "Yavin IV." The rebels had set up a base within the ancient alien ruins, taking advantage of the strongly built stone structures. The ceremony you see is inside the rebel base in probably the nicest room in the entire temple.

I actually didn't know about Yavin IV until I read the novel version of the film, and took a closer look in the movie. Though I think it was a little cheap for Lucas to have used distance shots of ancient Mayan temples to resemble "alien" temples in the jungle on that moon.

Later Expanded Universe lore came up with an explanation as to the aliens that built those structures thousands of years before the story took place.

In fact, if you play Star Wars the Old Republic, you would have the option to visit Yavin IV during the Old Republic Era :D

Yavin 4

If you go by the film itself it could be any planet, or perhaps a large astroid.