It is written in episode 7 of Neon Genesis Evangelion that the First Impact was the collision that created the Moon 4 billion years ago.

Just after the scene with Gendou Ikari talking to a government type man, you can see a document entitled "Second Impact to so no ato no sekai" ("Second Impact and the world after Second Impact"). In this document it states that the First Impact happened "oyoso 40 oku nen mae" ("about 40 "oku" years ago").

1 "oku" = 100 millions, in Japanese.
40 "oku" = 4 billions.

Then it goes on to state that it was the colssion that caused the separation of the Earth and the Moon.

Also in the Redone Evangelion Episode 21, you can see a subterranean scan of Antarctica made on September 12, 2000 (the day before Second Impact), revealing the giant spheric underground cavity where Adam has been discovered.

This cavity was called "GF-01 - Codename: White Moon".

The scan also states:
*Possibly created at time of Giant Impact
*Surface structure clearly artificial

There is some real world truth to this, as it has been theorised that the moon was created when something hit the earth a long time ago and part of the earth and whatever hit it came togeather to form the moon.