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 (behind the stage - not where the presidential Vertibird lands) 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.


 * Note that you may not have to kill the sniper mentioned above as it is confirmed that you can simply disarm the bomb on the vertibird then allow Ranger Grant to end the speech early after you inform him that there was a bomb attached to the president's vertibird.