This yajna (ritual, sacrifice) is performed to commemorate a person's living to see 1000 full moons. Because the time between full moons averages 29.531 days, this happens some time after the completion of 81 years.
The yajna is conducted for the benefit of the individual. In this case, the individual is the Avatar of the age, and the yajna was performed for a greater purpose: to bring peace and happiness in the world and help redeem man from the cycle of birth and death.
The yajna, which was conducted in Hillview Stadium in view of thousands, had two phases: a preparatory phase and an executive phase. In the first phase, there are various rituals, with holy ablutions being offered to Lord Shiva at the end. One part of the executive phase includes the individual occupying a throne positioned in a prominent place —hence Sathya Sai Baba on the Golden Chariot.
In these three videos, you see parts of the rituals, with many priests involved, Bhagavan's interactions with them, and shots of the crowds in Hillview Stadium. The third video shows Swami on the Golden Chariot.
The first video, above, was filmed on 15 November.
Read a more detailed description of this momentous event here: sssbpt.org/Pages/83-birthday/spc-overview.htm.