These are called bid parties, and they are hosted by fan groups who are trying to get the Worldcon hosted in their city someday. For the uninitiated, the Worldcon is more properly known as the World Science Fiction Convention. If you think Minicon is big, you haven't seen anything. Well, you can play too. The first trick is picking a year.
Eligibility is limited on one region per year, and they cycle through from East to West to Central.
The current cycle is shown in the second sidebar. International bids are eligible anytime, without upsetting the cycle. If the Worldcon is overseas then we hold something called the NASFiC, or North American Science Fiction Convention. NASFiC is held in the region that otherwise would have hosted the Worldcon, had it been on this continent. For instance, the 1995 Worldcon will be in Glasgow, Scotland, and since this is an Eastern year, the NASFiC will be in Atlanta that same year.