http://www.gaydaysanaheim.com/html/history.html
From the above site:
"Years ago, Disneyland used to have a private party one night of the year for gays and lesbians. When the event was canceled in 1998, we created Gay Day Anaheim. We based the happening on our big sister event in Orlando, Gay Day, which attracts over 100,000 gays and lesbians from around the world each June.
The first Gay Days Anaheim in 1998 attracted about 2,500 people. In 2005, we had about 30,000! Pretty amazing. What was once a small one-day gathering has become a full weekend event with parties, gatherings, group photos, scavenger hunts, private meals, and much more!
Remember, this event isn't sponsored by Disney (hence why it's "unofficial") so if you call Disneyland asking for information about Gay Day, they will tell you that this is not a Disney event."
That pic was taken two years ago!