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The origins of this idiom.
The meaning of this phrase.
Phrases and Idioms - The Meanings and Origins For Many Common Idioms and Phrases
To spoil someone's fun or plans; ruining a pleasurable moment
This phrase looks to have originated from a popular 1964 song called "Don't Rain on my Parade," written by Bob Merrill and composed by Jule Styne. It was sung during the musical Funny Girl. Due to the fact that I am unable to find the phrase in writing before this time, I've concluded this song must be where the expression came from, and if not, then it at least popularized the term.

There are several examples from 1964 that are talking about the song. For instance, in the Cumberland Evening Times, it says:

"Composer Jule Stype and lyricist - Bob Merrill are at the top in their specialities, and they have
provided several good songs . . . But they have let Miss Stresiand down in two vital numbers,
'Don't Rain on my Parade' and 'Who Are You Now?' with the result that the show suffers."

Shortly after its debut, the name of the song began to be used in a figurative sense by people, as seen in Mademoiselle from 1969:

"And the next day, when five members of Parliament drop into Boston unexpectedly, and the sound
system for a dark dedication is lost en route, and it looks like it might rain on her parade, she'll need
that extra time."
* Not to rain on your parade, but Bill is sick at home and won't be making it for dinner.

* Jake, I do not want you to be raining on my parade and spoiling what happens at the end of the movie; I want it to be a surprise!
Note: The origins for most idioms are unclear. Often times, the origins you see listed are plausible theories to how an idiom came to be, but not necessarily so. The quotes you see that contain the phrase are the oldest that I could find, but it's very possible there are older recordings somewhere, so if you know of any, let me know!

Keep in mind, just because you see a saying in a newspaper from 1850 does not mean it originated in that year, or from that newspaper. In all likelihood, if a saying is already being used in a form of media like that, it's probably from an earlier time. The purpose of these old quotes is to show, with proof, how old some phrases go back in history.
Title picture for the idiom Rain on Parade.
Know Your Phrase -  An Alphabetical List of Common Phrases, Sayings and Idioms With Their Meanings > R-Letter Expressions