about the forthcoming "ReiWa" nengo announced April 1

Wikipedia already has posted the update:

https://en.wikipedia.org/wiki/Japanese_era_name [cf. in Japanese, https://ja.wikipedia.org/wiki/%E5%85%83%E5%8F%B7]

"The new era Reiwa (令和)^[1] is expected to start on 1 May 2019, the day when Emperor Akihito's elder son, Naruhito, is expected to ascend to the throne as the 126th Emperor of Japan"

Not everyone knows that the name of each era does not have to match the years of the emperor or empress' reign, as in recent periods.

The era-name was changed when natural disasters spoiled a given period, for example. By switching to a more favorable name, the idea was to change the fortunes of people's lives. Looking again at Wikipedia there is fuller discussion of the concept.

Historical nengō

Prior to the Meiji period, era names were decided by court officials and were subjected to frequent change. A new era name was usually proclaimed within a year or two after the ascension of a new emperor. A new era name was also often designated on the first, fifth and 58th years of the sexagenary cycle, because they were inauspicious years in Onmyōdō. These three years are respectively known as kakurei, kakuun, and kakumei, and collectively known as sankaku. Era names were also changed due to other felicitous events or natural disasters.

In historical practice, the first day of a nengō (元年 gannen) starts whenever the emperor chooses; and the first year continues until the next lunar new year, which is understood to be the start of the nengō's second year