Today is the first day of the Golden Week holidays, Showa no Hi or Showa Day. It is the birthday of the former Emperor Hirohito (grandfather of the current Emperor). During his lifetime, his birthday was celebrated as a national holiday, as are the birthdays of all Emperors during their reign. When he died in 1989, April 29th became the Greenery Day holiday. In 2005, Greenery Day was moved to May 4th and April 29th was changed back to the day remembering the Showa Emperor.
Emperor Hirohito was the reigning Emperor from 1926 - 1989, before, during, and after World War II. The purpose of the holiday is to reflect on Japan's Showa period when the nation recovered after turbulent times and to think about the country's future. You can read about his life on Wikipedia here.
I don't know how much people really reflect and think. In normal years, it is a time to travel. This year, because of the ongoing pandemic, there is a lot of pressure to stay home over the holiday week. Very few people have been vaccinated so far and the new variants are making the news as being more contagious. I won't be traveling anywhere, but I might be walking around my neighborhood.