What concert does Leslie organize after Pawnee merges with Eagleton?

After Pawnee merges with Eagleton, Leslie organizes the Unity Concert.

The concert is planned to celebrate the merger of the two neighboring Indiana towns. Pawnee and Eagleton have long been rivals, and the merger forces their residents and government departments to work together despite years of mutual contempt.

Leslie, Ben and Tom seek sponsors, while Andy and Tom help develop the concert idea. The event becomes a recurring storyline in season 6: the characters search for performers, deal with the wall dividing the communities and struggle to persuade skeptical residents that a shared celebration can work.

The Parks Department ultimately holds the Pawnee-Eagleton Unity Concert in the season-six finale, “Moving Up,” and it succeeds. The answer is not “Harvest Festival,” which was an earlier Pawnee event, or “Pawnee Prom,” which Leslie revives in a separate episode after it is cut from the budget.