Step 1: Understanding the Concept:
United Nations Day marks the anniversary of the entry into force of the UN Charter in 1945, which officially established the global organization.
Step 2: Detailed Explanation:
United Nations Day is celebrated annually on 24 October. The year 2025 marked a historic milestone—the 80th anniversary of the UN's founding. To commemorate eight decades of multilateral diplomacy, human rights advocacy, and peacekeeping, the global theme chosen was "The United Nations at 80: Building our future together" (often stated globally as "Building Our Future Together"). The milestone focused on reforming international cooperation to better handle modern digital, climate, and developmental challenges.
Step 3: Final Answer:
UN Day is observed on 24 October, and the 2025 milestone theme was "The United Nations at 80: Building our future together".