In Japan, we use Gregorian era as like in western Europe and also imperial era. 2019 is Heisei 31th. Name of the era changes each time when the new emperor was ascend to the throne. Which means when the emperor abdicates or dies, it will be the last day of the era, and new era will start with the new emperor.
Current Japanese emperor Akihito will step down on 30 April and the new emperor will be his eldest son, the crown prince - Norihito. The current name of the era is Heisei and the next will be Reiwa. 01 May 2019 will be the first day of the new era - Reiwa 1st.
Akihito will be the first emperor in 200 years to abdicate, which means retire before the death. The last emperor to abdicate before his death was emperor Kokaku in 1817. Hirohito, Akihoto's father had an era named Showa. Then, today was the birthday of Hirohito. Birthday of Hirohito, 29 April is a public holiday in Japan as Emperor's birthday even after his death in January 1989.
However, the name had been changed to Greenery day. Then, in 2007, Greenery day moved to 04 May and 29 April renamed again to Showa day. Showa day will be kept as the public holiday but Akihito's birthday - 23 December will not be kept as public holiday anymore after the abdication. Which means 23 December 2019 will not be a public holiday. Due to the next emperor's birthday is 23 February, 2019 was the first year when there is no emperor's birthday since the introduction of new law after WWII.