Actually, Congress originally put tax day on the calendar. When the 16th Amendment, which allows Congress to institute the income tax, was adopted on Feb. 3, 1913, Congress chose March 1--one year and a few dozen days later--as the deadline for filing returns. Then, with the Revenue Act of 1918, Congress inexplicably moved the date forward to March 15. The next overhaul came in 1955, when buried between tax-code revisions was yet another date change, this time to April 15. According to an IRS spokesman, the move "spread out the peak workload," but there's another explanation. Turns out that as the income tax applied to more of the middle class, the government had to issue more refunds. "Pushing the deadline back gives the government more time to hold on to the money," says Ed McCaffery, a University of Southern California law professor and tax guru. Still, the IRS's rigidity works in your favor: By law, it must mail your refund within 45 days or pay you interest.
This isn't quite as entertaining as the explanation for Election Day—a story I'll tell some other time—but this explanation does make sense.