3/15 was originally tax day.
"They switched income-tax day from March 15 to April 15, thus giving the taxpayer an extra month to recover from Christmas expenses and sparing him the yearly ordeal of hearing and reading clichés about the ides of March."
http://time.com/3772455/april-15-history-tax-day/
"…1942, shortly after Pearl Harbor. The entire nation was mobilizing for war, and money was desperately needed. At the time, the idea of running a deficit was seen as disastrous…To pay for the war, Congress passed a new Revenue Act that nearly doubled the number of Americans who would have to pay income taxes. TIME called it 'the biggest piece of machinery ever designed to separate dollars from citizens.'"
http://time.com/4289687/1942-tax-day-history/