Which UN observance is held on 21 June after a proposal noting the Northern Hemisphere's longest day?

The UN observance held on 21 June is the International Day of Yoga.

India’s Prime Minister Narendra Modi proposed an annual yoga day in a September 2014 address to the United Nations. He highlighted 21 June because it is the longest day of the year in the Northern Hemisphere and carries cultural significance in several traditions.

The United Nations General Assembly adopted the observance on 11 December 2014. The resolution had 177 co-sponsoring countries and was adopted without a vote. The first International Day of Yoga was celebrated worldwide on 21 June 2015.

Yoga originated in ancient India and is promoted through the observance as a practice associated with physical, mental, and spiritual well-being. The date is not the same as International Dance Day or World Health Day, and the UN name is specifically “International Day of Yoga,” not merely “Yoga Day.”