Rick Monday was playing center field for the Cubs against the Los Angeles Dodgers in Los Angeles when 2 individuals come onto the field with an American flag. When it becomes apparent to Monday that they were planning on burning the flag, Monday runs towards them and grabs the flag after the two had doused the flag with lighter fluid and were about to light a match.
When Monday came to bat, the video board at Dodger Stadium "Rick Monday......You Made a Great Play"
Monday was presented with the flag on May 4th of 1976 by MLB commissioner, Al Campanis. Through the years, Monday has used this event to help raise funds for various veterans groups. He threw out the first pitch at Dodgers stadium and brought this historic flag back to the field 40 years after this event took place. It is listed as one of the 100 classic moments in baseball history.
Another interesting coincidence about this day was it was the 100th anniversary of the first game the Chicago National League team played (they weren't the Cubs in 1876, their name was the Chicago White Stockings at this time).
A very good video from MLB channel. It shows video of this incident, along with clips of Rick Monday discussing his thoughts about this incident
More interesting information from this video:
A good article from Tripping Baseballs on this incident. Nice photo of the Rick Monday flag rescue bobblehead.
In 2016, the 40th anniversary of Rick Monday saving the American flag at Dodger Stadium, Monday threw out the first ball and brought the flag he saved to the stadium.