Showa Era refers to the Japanese history time when Suzuno Osugi was inside The Universe of the Four Gods, following the Taisho Era. The era's end also marks the summoning of the beast god Byakko, along with the tragic separation of Suzuno and Tatara.
The Showa Era coincides with the reign of Emperor Showa, known to Westerners as Hirohito, who was in the throne from December 1926 to his death in January 1989. Socially and politically speaking, the Showa Era is marked first by the Japanese Empire's cruel, iron-fisted colonialism and expansion through South-East Asia and its extremely conflicted role in World War Two, and from 1945 on by the rebuilding of the country after both the atomic bombings and the American occupation.
Suzuno's adventures properly began in the Taisho Era, more exactly in 1923: her 8-year-old self was sent to the Universe of the Four Gods right after the Great Kanto Earthquake that destroyed Tokyo, returning to the ruined city few hours (book)/days (real world) later. Suzuno's actual adventures in the book begin in 1933, in the seventh year of the Showa Era: in these days, she is 17-18 years old and in her last year of high school. By that time, her adoptive brother and prospect love interest Seiji Horie is 22 years old and has entered the Imperial Japanese Army Academy (Rikugun Shikan Gakkou), intending to pursue a military career to either begin an independent life or to help Oikawa support the family.