表达 “light up like a Christmas tree（像圣诞树一样被点亮）” 有两个用法。第一：它可以用来形容 “某人突然喜笑颜开的神情”；第二：它可以用来描述 “某个地方的灯光非常夺目”，暗示夸张的照明。

例句
When I gave my son his present, he lit up like a Christmas tree!
当我把礼物送给儿子时，他顿时满面笑容！

He’s planning to propose to his girlfriend today. He’ll light up like a Christmas tree if she says yes.
他打算今天向女朋友求婚。如果她答应了，他会高兴地手舞足蹈！

They’ve put so many garden lights on their lawn, it looks like an airport runway. It’s lit up like a Christmas tree.
他们在自家草坪上装了很多庭院照明灯，看起来就像机场跑道一样，灯光效果十分夸张。