Gengou, or the name of the era, it's length defined by the ruling of each Emperor. Up until April 30, 2019, was Heisei, ruled by Emperor Akihito, before that Showa, which went until 1989 under Emperor Hirohito. In 2016, Emperor Akihito announced his request to step down, creating a wave of commotion in Japan, since traditionally, an Emperor is an Emperor until his death. However, a ruling was decided for an exception to be made to let Emperor Hirohito step down, and for the Crown Prince Naruhito to take the throne, officially changing the gengou from Heisei to Reiwa.
Japan celebrated the start of Reiwa a little earlier than us. And boy, did it come with celebration. There were countdowns around the country, products such a Coke bottles, cakes, and keychains were being sold with Reiwa printed on them, and overall was a marketing opportunity for companies, as well as, an excuse for citizens to bask in the joyous mood. The transition from Showa to Heisei back in 1989, when Showa Tenno (Emperor Hirohito) passed, came with a less celebratory mood, as the country mourned the loss of an emperor. The beginning of Heisei came with sudden anxiousness riddled with the relief of the end of an era that was painted with war.
Many Japanese citizens have an identity attached to the gengou they were born in. While it's "just a year" any other day, parting with something you've gotten used to has become a bittersweet thing for many Heisei born folks. It's almost how Americans are proud to be a "90s baby", attributing an era of culture and history to when they were born.