Clearly define what the event is and what your expectations are.
Include important information: agenda, ticketing, and more. Users only need one place to go for everything. Is the event a wedding, a family reunion, a movie night? The possibilities are endless! And as with any site, the purpose needs to be clearly defined.
Time is the only constraint…
As this is a one-time event, time becomes the biggest obstacle. Start early and keep in mind the event’s date to maximize the potential of your event site.
Explore examples of sites that are created for one-time events and get a feel for options that are available.
Source: You are browsing 207 One Page Event Websites One page Event website designs. These are great references for how to provide valuable information while still creating hype (buzz) for the event. See One Page Event Templates.