Ladies and gentlemen, I'd planned to speak to you tonight to report on the state of the Union, but the events of earlier today have led me to change those plans. Today is a day for mourning and remembering. (1-2)

The first two sentences are totally not famous, but they do a lot of work for only being two puny sentences.

The "ladies and gentlemen" intro is formal, though it's formal in a predictable way. It very gently and respectfully invites people to pay attention. After a day of defective rockets and group death, a little bit of old-fashioned etiquette was probably a welcomed gesture.

It sets a solemn tone that is sustained throughout the speech and is emphasized by Reagan's declaration that the day is "for mourning and remembering." Besides, opening with a knock-knock joke wouldn't have played too well to the audience (the grieving population of an entire country is a tough crowd).

Reagan makes mention of the rescheduled State of the Union address in a manner that is almost offhand, but this nonchalance further stresses the gravity of the Challenger Disaster and the importance of what he is about to say. Some things are bigger than a lil' SOTU talk.