What is a Calendar Queue?

I am working on a building a discrete event simulator. Wikipedia mentioned that there are several general purpose priority queues that are good for use in DES's. Specifically, it mentions that a Calendar Queue is a good structure. I found one pdf (from 1988) that mentions Calendar Queues, but for the most part I can't find anything else out about them. Would someone mind explaining what Calendar Queue's are, how they're used, and where I might find a sample implementation?