五十日

Etymology 1
Compound of.

The hi changes to bi as an instance of.

Noun

 * 1) the days of a month that are multiples of 5 or 10 :

Etymology 2
Compound of.

Noun

 * 1) fifty days
 * 2) * c. 1028-1107,  (volume 1, page 32)
 * 3) * c. 1080s,  (volume 2, page 182)
 * : a celebration on the 50th day after a child's birth
 * : mochi fed to the child during ika no iwai
 * : a celebration on the 50th day after a child's birth
 * : mochi fed to the child during ika no iwai