rain on (one's) parade
To ruin one's plans or temper one's excitement. I hate to rain on your parade, but I think your A in chemistry was actually a clerical error. Mom really rained on our parade by chaperoning our school dance.
rain on one's parade
Spoil one's plans, as in The minority party in the legislature has tried hard to rain on the speaker's parade, but so far his agenda has prevailed . This expression conjures up the image of a downpour ruining a celebration such as a parade. [c. 1900]