What is Labor Day and Why Do We Celebrate It?

What is Labor Day about and why do we celebrate it each year as a national holiday?
I just want to know the meaning and significance of Labor Day weekend 2011. It means the end of a summer and is a free weekend but I wonder what it means.
I remember one of my teachers once told us what it is but I forgot all about it.
So what is it celebrated for this year?

Labor Day has been celebrated as a national holiday in the United States since 1882.
The holiday originated as a result of the labor movement and is a day of rest to recognize the efforts of the average working man and woman.
Labor Day constitutes a yearly national tribute to the contributions workers have made to the prosperity of our country.
The first state to enact a bill that would eventually become law to celebrate Labor Day was Oregon on February 21, 1887.