When is it moved to? April, 16th is normally Emancipation Day, an official holiday for the District of Columbia. This year that holiday will be celebrated on the 15th of April, due to our leap year. Therefore the new deadline is April the 18th the following Monday after the holiday.
Unsure of this holiday? According to timeanddate.com "Emancipation Day is a holiday in Washington D.C. to mark the anniversary of the signing of the compensated emancipation act, which president Abraham Lincoln signed on April 16, 1862. It is annually held on April 16."
Under IRS rules they consider any holiday celebrated in D.C. a nationwide holiday hence, moving the deadline.