Need to have an important talk with someone? Right now is a very bad time to bring up the following, so wait at least a week, mull it over, and then you can have a more positive, productive discussion...
Moving. Whether you want someone to move in or move out, avoid making any decisions or even bringing it up. You are likely to change your mind or scare the other person away.
Making a big purchase. Now simply isn't the best time. If you can put it off, think it over, or even shelf the idea permanently, something better is going to be available later.
Criticizing an annoying habit. If your roommate, partner, colleague, or friend is doing something that gets on your nerves, try to just remove yourself from it or block it out. Don't call them out right now because it will only get worse.
On all of these, though, if your heart and head agree that something needs to be said or just feels SO right, trust your instincts and they won't fail you!