Simplify the following text for a Grade 7 native English-speaking person.
Original: On May 19 , 1962 , Monroe made her last significant public appearance , singing `` Happy Birthday , Mr. President '' at a birthday party for President John F. Kennedy at Madison Square Garden .
Simplified: On May 18 , 1962 , Monroe made her last major public appearance , singing `` Happy Birthday , Mr. President '' at a birthday party for President John F. Kennedy at Madison Square Garden .