also: じゅんじつ
leap day
Refers specifically to February 29th in a leap year.
Leap day appears only once every four years.
Alternate on'yomi reading, less common.
Mixed kanji-kana spelling, common in everyday writing.
閏 (intercalary) + 日 (day). The reading うるうび is a kun'yomi compound, while じゅんじつ is on'yomi.