Today April 29 is the national holiday Showa day in Japan. It honors the birthday of Emperor Shōwa (Hirohito), the reigning Emperor before, during, and after World War II (from 1926 to 1989). The official purpose of the holiday is "To reflect on Japan's Shōwa period when recovery was made after turbulent days, and to think of the country's future."
Before the Emperor died in 1989 is was a day to celebrate the Emperor's Birthday. After he died the day became Greenery Day which has been moved to May fourth in 2005 (so Ranma time this would have been Greenery Day).
It is part of Golden Week. Golden Week is a collection of four national holidays within seven days. In combination with well placed weekends, the Golden Week becomes one of Japan's busiest holiday seasons. This Year the weekends fall in a way that results in a Golden Week consisting of an isolated public holiday (Today) and a 4-day weekend (May 3-6).