To rain on someone's parade is to spoil their enjoyment of their moment. Not to rain on your parade, sorry to rain on someone's parade, they rained on her parade.
It is nearly always used as an apology in advance, since saying you do not want to rain on someone's parade means you are about to. The image needs no explaining.
1phraserain on someone's paradeTo spoil somebody's enjoyment of their moment.
“Nobody wanted to rain on someone's parade by mentioning the cost of the wedding.”
rain on someone's parade is a spoiling.
An American phrase from around 1900, from the plain image of rain spoiling a parade.
First appeared around 1900.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms