Shirozake is a case in point where a drink is call sake while it is not Nihonshu or Seishu. That is it is not the refined sake we think of as sake in the US.
Shirozake was created sometime around 1600 to 1650 when the founder of Toshimaya, a sake merchant and food company, had a dream in which a paper doll told him how to make shirozake. He carried out the instructions producing the first shirozake.
Shirozake was very popular through the 1800s. For example in 1880 270,000 liters of shirozake were sold. Shirozake became tied to the Hina Matsuri (雛祭り) or Doll festival (March 3rd) where it is mostly drunk by women. In order to meet these high demands, Toshimaya would focus exclusively on shirozake sales from around the end of February each year.
Shirozake is a sweet white sake like drink made by combining rice, koji and shochu to form a liquor. To make shirozake the rice is steamed and mixed with koji and shochu and then left to age for a month. Once aged the mixture is puréed into a consistently smooth drink about 45% rice extracts and having 8-9% alcohol.
Toshimaya Shuzo in Tokyo still sells shirozake after 400 years though there are other producers now.