The fifth 節気 “sekki” division of the traditional Japanese calendar is:
pure and clear, clear and bright (solar term)
The third and final “kō” microseason within this division is:
(にじ はじめて あらわる)
“niji hajimete arawaru”
First [sight of] rainbows
That makes 虹始見 the fifteenth microseason of the year!
It typically lasts 15~19 April.
beginning, start, outset
Image credit: Jeffrey Friedl’s Blog.