You'll Know It When It Happens

 is a quest in Fallout: New Vegas.

Quick walkthrough
In order to save the president you must be on the guard tower closest to the president's stage where a Ranger sniper is. Soon after the president starts speaking a Legion member dressed as a Ranger will come up through the hatch and kill the Ranger Sniper posted there, that is when you should kill him. After doing so, use the radio and report that a Legion member has been killed. Another assassination attempt will be made when a man in Engineer clothing runs up to the stage and kills the Ranger telling the President to get out. Kill the engineer from the tower before he can kill the President.