Japan: Registered cover sent from Okinawa

My friend Li Ming sent me also registered cover from Japan, from Shiraho on Okinawa Islands. There's a nice First day postmark, but i don't know what's the occasion. He used seven different stamps on this cover:

Prefectural Stamps - Okinawa, issued on 21.6.2000.
World Performing Arts Festival 2000, issued on 28.6.2000.
Letter Writing Day, issued on 23.7.1987.
Letter Writing Day, issued on 23.7.1986.
International Year of the Elderly, issued on 1.10.1999.
World Aeromodel Championships, Kasaoka, issued on 25.9.1995.
The 10th Anniversary of Accession of Emperor Akihito, issued on 12.11.1999.


Thank you Li Ming!