Today is a national holiday in Japan. Actually, it is THE national holiday in Japan, called Kenkoku Kinen no Hi, National Foundation Day. It is meant to celebrate or commemorate the day when the first emperor of Japan – Jimmu Tenno – ascended to the throne.
However, the whole thing is a little bit more complicated than that. It starts with the fact that emperor Jimmu is more a mythical creature than a real person. He is said to be the direct descendant of the sun goddess Amaterasu and was the first to unite all the clans of Japan under one rule. Traditionally, his ascention to the throne is said to have happened on the first day of the lunar calendar in 660 BCE, but with the switch to the solar Gregorian calendar in 1872, the day got moved around a bit until it was finally settled to be celebrated on February 11.
Before WW II, Kigensetsu, as it was then called, was a way to celebrate the Japanese nation and empire and its ruling family. As such it had a deep nationalistic meaning and was promptly outlawed after the war when overt displays of nationalism were discouraged by the occupying forces. The day was reinstated as a national holiday in 1966 and then renamed to Kenkoku Kinen no Hi. Since then, it is a minor holiday, and although the hinomaru, the national flag, is ceremonially raised on government buildings, the celebrations overall are quite subdued.