Sasaki, Sanmi (佐々木三味); McCabe, Shaun; Iwasaki, Satoko (2005). Chado the Way of Tea: A Japanese Tea Master's Almanac. Tuttle Publishing. ISBN9780804837163., p.372 describes Doyo as "about eighteen days from July 20th", though in actuality the beginning date can shift a day or two. The days cycle through the twelve animals (twelve heavenly branches) so any one or two of the dates within the eighteen-day period can fall on an Ox day.
Heibonsha 1969, vol. 7,"kabayaki" by Tekishū Motoyama (ja:本山荻舟) describes it as being used principally or almost always for unagi (「もっぱら鰻」) Heibonsha (1969) [1968]. 世界百科事典(Sekai hyakka jiten). (world encyclopedia, in Japanese), vol. 7, article on "kabayaki" by Tekishū Motoyama (ja:本山荻舟) (1881-1958).