That's because of the "Saturday, Sunday, holiday rule," which says that if April 15 falls on a Saturday, a Sunday, or a legal holiday, Form 1040 is due on the next day that is not a Saturday, Sunday, or legal holiday. You might not have known that April 16 is Emancipation Day, a legal holiday in Washington, D.C. that commemorates the signing of the Compensated Emancipation Act by Abraham Lincoln on April 16, 1862. But this year, April 16 falls on Saturday, so Emancipation Day is being celebrated on Friday, April 15. Therefore, Form 1040 is not due until Monday, April 18.
By the way, if you make quarterly estimated tax payments, that's not due until Monday either.