A number of conventions (e.g., Worldcon, Westercon, and Eastercon) move from place to place with convention sites and committees being selected one or two years in advance.
For example, to win the opportunity to host a Worldcon a group of fans organize a bid committee which starts bidding 2-3 years before the site selection vote which will select the convention. For a Worldcon to be held in 2020, the vote (under current rules) would be held in 2018, so bidders would announce around 2016. Typically, the bid would be named "XXX in 2020", where XXX is the name of the city being bid. (E.g., Boston in '89, KC in '76, Chicago in 2012.)
Bidders run bid parties and host bid tables at regional conventions and at Worldcon, run ads in selected regional program books, and generally get their name and story out to potential voters.
Most years there is either a single bidder or two bidders, but even if there is only one bidder, an active bid campaign is essential because the goodwill points gained by a well-run bid translate into an eagerness to attend the convention and — especially with Worldcons and their high fixed costs — a few more members translates disproportionately into more money to put into niceties at the con.
One means of gathering support is to sell various forms of pre-supporting memberships.
