lit up like a Christmas tree

Lit up like a Christmas tree is a phrase that originates from the military when they blow up an enemy position using bombs, bullets and missiles. This is a way to show that with all of this blowing up of bombs and missiles, the enemy line lights up just like a Christmas tree. When used figuratively, it can be in reference to anything you do causing something to light up like a Christmas tree but are most commonly used literally such as fireworks or other things that light up in the same way.