It brings the various components of a regional biodiversity conservation strategy together with hopes they will network together to save the region's genetic material. Find out who the contact people are and call them and ask if they will come with a display for the day. Ideally everyone donates their time to the cause. You will need tables for people to set up displays and some chairs behind the tables for the display people. Ideally no admission charge, or an 'admission by donation' policy so no one can be prohibited by attending by admission costs.
Parking is a concern and bus accessibility.
Decide when to hold the event. Spring is good, as in some areas folks divide perennials and bring them to the swap. In some areas scion wood is also sold or swapped or tubers or rhizomes sold or swapped. The timing of the event determines how many people you attract. Stanley Cup weekend, Easter, large social event in town not good times.