Complex sentence: The next day, on April 5,1866, the Senate overrode President Johnson's veto, the first time it ever overrode a president's veto for a major piece of legislation.
Split into simpler sentences: The next day, on April 5,1866, the Senate overrode President Johnson's veto. This marked the first time that the U.S. Congress ever overrode a president's veto for a major piece of legislation.