Congrats...you've now earned enough to pay all your taxes. From here on in, you're working for yourself - not government.
From their website:
Tax Freedom Day is calculated by dividing the official government tally of all taxes collected in each year by the official government tally of all income earned in each year. Governments—federal, state and local—took 30.4% of income in 1980; 30.4% in 1990; 33.6% in 2000; and so on. This percentage is what is properly called the "nation's total tax burden." We then use the historical trend and the most recent economic data to make a projection of what the tax burden will be in the current year.
Does Tax Freedom Day measure the "average" American tax burden?
Yes. The mathematical average of any group is found by adding up all the values and dividing by the number of values. Tax Freedom Day totals up tax collections and divides by national income. The result is the average tax burden for the U.S. economy as a whole. Including high-income taxpayers tends to increase the average tax burden—just as including low-income taxpayers decreases it. To be an objective mathematical average, Tax Freedom Day must include all taxpayers.