一代一度

いちだいいちど

no-adjective, noun

•

once in an emperor's reign, event that takes place only once in a sovereign's reign(yojijukugo)

一	one, one radical (no.1)
代	substitute, change, convert, replace, period, age, counter for decades of ages, eras, etc., generation, charge, rate, fee
度	degrees, occurrence, time, counter for occurrences, consider, attitude