expression, noun, adverb
once a year
A fixed expression meaning 'once per year'. Used both as a noun and adverbially.
このイベントは年に一度開催されます。
This event is held once a year.
年に一度の健康診断を受ける。
I get an annual health checkup.
年一回 is a more formal or written synonym for 'once a year', often used in schedules or statistics.
Compound of 年 (year) + に (particle) + 一度 (once). The pattern 〜に一度 means 'once per ~'.