What is an SF convention?
I've been asked that question many times, both rhetorically and seriously. So what follows is my attempt to answer it. It's possible I've left stuff out, since there are parts of Conventions that I don't get involved in much.
At the simplest level, an SF convention ("con") is a gathering of SF and Fantasy fans, usually held over a weekend in a hotel or convention centre.
Typically, the earliest arrivals show up at some time on the day before the convention, with the bulk of the attendees arriving sometime during the afternoon/evening of the first day of the con. Many people will depart during the afternoon of the last day, though there are usually enough left to hold a "dead dog party" during the evening.
If the convention is in a holiday-friendly location (such as Jersey, site of the 2002 Eastercon), then significant numbers of people are likely to show up earlier and leave later. This leads to bizarre context clashes as you keep bumping into people you normally only see at conventions while wandering around the local tourist attractions.
Most cons will have one or more Guests Of Honour (GOHs). Numbers will vary, depending on the size of the con, and they will be authors, artists or prominent fan personalities. If you want to meet the cast of "Star Trek: Yet Another Series" or the latest Star Wars film, this is the wrong sort of con. Any convention publicity material will tell you who the GOHs are, once the convention is confirmed as going ahead. Unlike at many media-oriented conventions, most GOHs will spend a lot of their time behaving just like the other attendees - going to programme items, lounging in the bar, etc. In fact, it's pretty common for famous authors to show up as regular attendees - not least because they are fans too.