There is an actual website devoted to giving the information of the next mooning for Amtrak trains. It's been a tradition for 33 years! They have also been doing an annual mooning of the Metrolink for 7 years.
Everyone can meet up along the chain link fence across from the Mugs Away Saloon in Laguna Niguel, California. The tradition lasts all day. The website gives driving directions to the spot and suggests that you arrive by the Metrolink, because it is too congested to come by car.
They ask for volunteers to help set up, but no one is specifically in charge. The Mooning Day is supposed to be a fun and law abiding tradition. The sheriff does make his rounds, so no alcohol or genital showing is allowed. They even have ‘night mooning’ after 8pm. Don’t miss the festivities!