Do you know when – and why?
Answer:
Ronald Reagan was the only president to have postponed his State of the Union address.
On January 28, 1986, he planned to give his address, but after learning of the Space Shuttle Challenger disaster, he postponed it for a week and addressed the nation on the day’s events.
As an interesting side note, not a single justice of the Supreme Court was in attendance for this postponed address, the first ever such absence.