Originally a compound of 此(ke, apophonic form of ko, "this") + 日(fu, apophonic form of hi, “day”). The final /eu/ → /joː/ sound shift is a separate but regular phonological change.
Today fell spring rain, recalling catalpa bows that one bends to string. If it but falls tomorrow, we will be picking young greens.[5]
2013 April 7 [2012 December 9], ONE with Murata, Yusuke, “[11](じゅういち)[撃](げき)[目](め) [強](つ)さの[秘](ひ)[訣](けつ)”, in [ONEPUNCH-MAN](【ワンパンマン】), 6th edition, volume 2 (fiction), Tokyo: Shueisha, →ISBN, page 86: