He opens his speech with a sentence referring to “a date which will live in infamy" to grab his audience's attention.

He opens his speech with a sentence referring to “a date which will live in infamy" to grab his audience's attention.
Explanation:
The anecdote about the Japanese ambassador is important for the idea of the Day of Infamy as it points out the nature of the happenings in the Pearl Harbour. President Roosevelt refers to the diplomatic relations between Japan and the USA, as he was saying that the Japanese ambassador was communicating with the USA after the attack on Pearl Harbour was underway. The main idea of this speech is the Japanese trait of the trust of the USA.