I do a lot of web sites for organizations and candidates that have upcoming events. They are listed on the Upcoming Events page (in a list... no calendar-style grid) and a couple are listed on the sidebar of the home page.
I had been doing popups because there is generally not a lot of info (though not extremely common, it's possible that they have a bit of info), so it seemed not to warrant a full page for a little bit of text.
So another option is to just open a new page for that event, even if it is five lines long (perhaps it is worth mentioning that the events are maintained by clients via CMS, so long- and short-winded events play by the same rules).
Then another option could be to open up a hidden div (display:none -> display:block) with the details. This would probably work quite nicely on the Events page, itself, but not the home page sidebar. Perhaps the sidebar would link to the Events page where that one event would be expanded?
Seems like the div option has the least usability issues (if programmed correctly so it degrades nicely), the new page option is not really good for small descriptions (which lowers user trust in subsequent links, thus reducing usability), and popups are sooo 1996.
Am I missing anything? Anything better? Or have I answered my own question?