also: じゅんじつ
noun
leap day
Refers specifically to February 29th in a leap year.
閏日は四年に一度だけ現れる。
Leap day appears only once every four years.
閏年 means leap year, while 閏日 specifically means the leap day itself.
閏 (intercalary) + 日 (day). The reading うるうび is a kun'yomi compound, while じゅんじつ is on'yomi.