Sahasra purna chandrodayam is the celebration of a person's 1000th full moon during his or her life as a special occasion. This is a custom in India and Nepal. As the time between similar lunar phases, the synodic month, is on average 29.53 days, this is celebrated 3 full moons before your 81st Birthday. This ritual is also known as Sahasra Chandra Darshan (सहस्र-(पूर्ण)चन्द्र-दर्शन) or Chandra Ratharohan.
In Sanskrit sahasra means 1000, purna means full, and chandrodyam means dawn of moon.
A similar kind of celebrations for elderly persons are
References
Sahasra purna chandrodayam Wikipedia(Text) CC BY-SA