What does Nintendo events mean on a list of Pokemon for Pokemon Silver and gold?

A Nintendo Event is a special gathering or some other event hosted by Nintendo to celebrate something (ie, the release of a new game or movie, an anniversary, etc). Special Pokemon, such as Celebi or Mew, are inserted into games of the participants. For older games like Pokemon Silver and Gold, Nintendo no longer distributes Pokemon. Sorry.

When Nintendo was distributing event Pokemon for Gold and Silver though, the main Pokemon available was Celebi. The player had to travel through Ilex Forest to the Shrine to meet Celebi and catch it. As for Mew, it was to be traded from Red, Blue, Yellow or Green and the other Pokemon (Lugia or Ho-Oh) was available in the game anyway.