A few clarifications for windigo's writeup...
Yes, it was traditional to appoint or otherwise choose a May King and Queen at the start of the planting season, near or on Bealtaine as a matter of fact. And yes, the King and Queen were treated extremely well during the spring and summer, in the hope of a good harvest.
Where the history differs comes at the time of the harvest. If the harvest was decent, all was good, and the next year, a new King and Queen would be chosen. If, however, the harvest was not so good, then yes, the King and Queen would be sacrificed. There was nothing sadistic about this, however. Keep in mind that death was not feared among the sidhe and other ancient peoples as it is feared today. Death was merely seen as a passing on to another plane of existence. In this case, sacrificing the May King and Queen was done to send them on as messengers to the gods, asking them to explain why they didn't smile on the harvest this year, and to ask for a better one next year.
Hence the royal treatment the King and Queen would receive during the summer. If you were going to go talk with a god face-to-face, wouldn't you want to look your best and be in a good mood? And if you were responsible for sending that messenger, wouldn't you want the gods in question to see that you wouldn't send just anyone, but rather someone you thought of very highly and treated very well?