I have searched for some reports on the previous birthday's. Most of the information was found on the SL History Wiki.
On Wednesday, June 23rd 2004, Second Life turned a year old (a year from its initial launch date into "public" status of June 23 2003), and many events were held in its honor! The celebration spanned over several days and was also known as the "Second Life First Year Anniversary Gala". The main events included a "Gala Parade" featuring floats created by residents, a festival and kid avatar contest, and the Time Capsule event with a live broadcast by Philip Linden.
There were also a lot of unofficial events held by other residents through-out the week of the celebration. These events ranged from sales, to club openings, to parties and more.On June 23rd 2005 Second Life celebrated its 2nd birthday, or rather the day Second Life was opened to the public! The main celebration was held in the Vehicle Sims of Bethel, Brilliant, Georgean, and Fortuna. The four vehicle sims were turned into a giant museum displaying various large builds, pictures of SL over time, and objects from the time capsule opened last year. The main layout comprised of four large pathways going out in all cardinal directions with a large celebration area in the middle at the corner of the four sims. Along the sides of these pathways were the various builds and objects people laid out for others to see. The event area and event itself were all put together by a group of residents led by Pathfinder Linden who happily took on the task of setting it all up.
The SL 3rd birthday didn't really get all the attention it deserved, with all the CC-less sign up drama happening the last weeks. And you could see it weighing on the organizers, who must have spent all their free time on this. Philip's speech was completely sidetracked by these issues and the obnoxious griever.
What is interesting to see is, look at the number of residents, and the ones which were logged in at that time. (date June, 29 2006)
There was not much to find on the internet about last year’s event. Was it not successful? Although many of us were also present at the third birthday, it seems that no one has made a report of the official opening. The only reports I could find were on the mentioned links.