All five nations—Costa Rica, El Salvador, Honduras, Nicaragua, Guatemala—celebrate independence on September 15 because they gained sovereignty together.
On September 15, 1821, the Captaincy General of Guatemala (Spanish colonial administration covering the region) declared independence from Spain in Guatemala City. The act applied to the entire territory. No separate wars or declarations occurred for each province initially.
Each country now marks the date with parades, torch relays (Antorcha de la Independencia), cultural festivals, and civic ceremonies—shared history, shared celebration.