120th birthday
A celebration of reaching 120 years of age, twice the traditional 還暦 (60th birthday). Rare in practice, but used in longevity contexts.
People who reach their 120th birthday are extremely rare.
Compound of 大 (great) and 還暦 (60th birthday, from the traditional 60-year cycle). The exact origin is uncertain, but it follows the pattern of doubling the traditional milestone.