When you are looking forward to something constantly, to the point of obsession, it will cause the actual event to seem short and dull in comparison. This is called “Christmas Syndrome”.

你一直盼望某事的到来，甚至到了狂热的程度，结果这件事真的发生了，反而显得又短暂又无趣。这就是“圣诞节综合症”。

Example:

He's been counting down the days since July, and has came down with a bad case of Christmas Syndrome.

他从七月份起就开始数着日子盼望圣诞节的到来，结果郁闷地发现圣诞节过得如此匆匆，也没什么意思。