Sri Lanka gained Independence from Great Britain in 1948. Since then February 4th is celebrated all over the country, but especially in the capital. In Colombo, the President raises the national flag himself and gives a speech along with other public figures. The official ceremony is followed by a military parade. You can see tanks and aircraft. And also this might be one of a few occasions when you can see an elephant marching with the military during a parade.
The official celebration is followed by cultural performances, music and dances. Independence Day is a good occasion to see the culture of Sri Lanka at its best and brightest.