みどりの日 means Greenery Day, a national holiday in Japan.

Decades ago during the Showa era, there was a national holiday “Birthday of the Emperor” on April 29, which was the birthday of the Emperor Showa. After the Emperor Akihito’s ascension in 1989, since the Emperor changed, the holiday “Birthday of the Emperor” moved to December 23, the birthday of Emperor Akihito (and later moved to Emperor Naruhito’s birthday on February 23 in 2019). To prevent interrupting the Golden Week arrangments, the holiday on April 29 was renamed Greenery Day to acknowledge Emperor Hirohito’s love for nature without directly mentioning his name.

In 2007 after some legislation, Greenery Day was moved to May 4, and the day April 29 was renamed Showa Day (昭和の日), extending Golden Week such that less leave would be needed for those willing to take a prolonged vacation.

Like other holidays, it’s a hashtag with which users tweet more often on the day, causing みどりの日 to trend on Twitter in Japan.

Learn more on wikipeida about Greenery Day.