Before we get started, go hover over one of the pink squares in the calendar to the right of this article. That's what we'll be making. Webkit users, you'll notice the fading in effect (accomplished with a CSS transition). Firefox users probably won't, but I've seen rumors that Firefox is going to add CSS transitions in 3.7, so hopefully that's true.
The code I used for this is actually pretty straightforward: the calendar itself is a table with each day being a table cell, days that have entries associated with them have links in them (the pink boxes) these pink boxes then have a span containing the title inside of them -- this span is the tooltip. The span is then hidden until the link is hovered over.