With research contributed by Melissa Fernley.

As the old saying goes, the two things you can’t avoid are death and taxes. But while the grim reaper may arrive unplanned, it’s generally understood in the U.S. that the taxman comes calling on April 15th – except when he doesn’t. This year, today, April 17th, is Tax Day. And last year, in 2011, it was April 18th. What causes this variation in the tax filing deadline? And why is Tax Day April 15th (ish) anyway? Read on for answers to all of your tax questions (that don’t actually relate to your taxes).

As has become popular in so many other areas, we can blame Congress for the scheduling of Tax Day. When the 16th Amendment (which allows Congress to institute the income tax) was adopted on February 3, 1913, Congress chose March 1, 1914 as the first deadline for filing returns. Pursuant to the Revenue Act of 1918, Congress moved the date to March 15. Then, when the Tax Code was revised in 1955, Congress again moved the filing deadline to April 15. Theoretically, the move was intended to spread out the workload for IRS employees. Another suggested explanation is that delaying the deadline allows the government keep what will become refund money (withheld with the payroll tax) for longer, thereby letting it earn more interest while still in the government’s hands.

Since the start, when April 15th falls on a Saturday or Sunday, the deadline for filing returns is the following Monday. In 2005, however, the District of Columbia enacted Emancipation Day on April 16th, to commemorate the day President Lincoln signed an 1862 law freeing the first slaves in Washington, D.C. When April 16th falls on a Saturday, the holiday is celebrated on a Friday, and when the 16th falls on a Sunday, the holiday is celebrated on Monday. This explains last year’s April 18th Tax Day – the 16th was a Saturday, so Emancipation Day was celebrated on Friday the 15th, pushing Tax Day to the following Monday. This year, the 15th fell on a Sunday, and with Emancipation Day on Monday the 16th, Tax Day became Tuesday the 17th.

Ironically, April 17th is also “Tax Freedom Day” for 2012, the day on which the average American will have earned enough to pay his or her estimated share of all of the taxes the government will collect. Each year, the Tax Foundation, a nonpartisan tax research group, determines Tax Freedom Day by dividing the total of all federal, state, and local taxes collected each year by national income, and then multiplying the result by 365. For 2012, the total tax burden comes to 29.2% of income. Therefore, Americans must work for 107 days to pay their taxes. Skipping Leap Day to keep things comparable from year to year, the 107th day of 2012 is April 17th. Although Tax Freedom Day is later than it has been in past years, it was the latest in 2000, when it fell on May 1.

Some other fun Tax Day facts? The U.S. stock market tends to outperform in the two weeks leading up to the tax filing deadline, providing many with the liquidity needed to pay taxes. Since Tax Day became April 15th in 1955, the Dow Jones Industrial Average has gained 1.35% in the first two weeks of April, compared to the average two-week gain through the entire calendar year of just 0.25%. In addition, many businesses offer specials on Tax Day to try to lure in customers with fresh refund checks, or those who are feeling the sting of paying a tax bill. Visit this Huffington Post article for a list of some Tax Day deals.