Washington's Birthday is a United States federal holiday, celebrated on the third Monday of February in honor of George Washington, the first President of the United States.
George Washington said "I cannot tell a lie" so, in his honor, this President’s Day aim for the honesty of the expression “I don’t know”, if you don't.
Tip: Be sure to follow it up with what you will do to find out:
"I will research and get back to you by 3pm"
"I will transfer you to the person who takes care of that"