Stop waiting for the right time
Don't think too deeply about when—the right time is when you know it's over. Do it then; don't wait. Don't wait until the summer just to keep a warm body in your bed, or until a holiday passes, or any other arbitrary mark on the calendar.
Sometimes the worst times are actually the best times, in retrospect. For instance: Breaking up with someone after arriving home from a trip from Vegas might seem like the worst possible time (oh, the implications!) but it can actually prove to be the best (your things are already packed!).
Yet another example: Right before Valentine's Day seems like a bad idea, (who willingly becomes single then?) but it gives you the opportunity to drop a great one-liner like, "I'm giving you up for lent" (which was the following week). Yes, these things happen.
The point here is that when you know you don't love someone anymore, there is no right time to tell them. However, there is a wrong time. And that wrong time is weeks or months after you've already made the decision.
Carrying on the charade of affection is hard work, not to mention cruel. You're not being kind to wait for "the right time," you're just prolonging the inevitable. Stop procrastinating.