O que significa "to rain on your parade"?

Imagine you've been planning your next weekend for quite a while. You've decided which place to go, which hotel you're staying in and all your friend are looking forward to it because they're also coming along. There's really too much expectation going on about that trip. Two days before the weekend though your boss comes up to you and says:

I'm sorry to rain on your parade, but this weekend you're going to have to work overtime.

Can you imagine what just happened here? It's clear that your boss came up with bad news, but what did he exactly mean by saying to rain on your parade? He meant he was sorry for ruining your pleasure or plans. Until he spoiled them, you couldn't believe how much fun you were going to have with your friends in a couple of days. Now, however, it seems your boss has brought a dark cloud over your parade and made it rain on it. There are some people that seem to get pleasure out of that. In a previous post, I mentioned people like that are known as party-poopers. They always ruin a festive moment by getting in the way.

But there are also some moments you don't really want to ruin someone's joyfulness purposely, but you know you've got to. There are some places for instance whose policies can't allow people to do certain things. If you want to smoke in closed spaces these days, someone may warn you by saying: I'm sorry to rain on your parade, but you're not allowed to smoke on the premises.