Nativity of the Theotokos is one of the Twelve Great Feasts of Eastern Orthodox liturgical year. Orthodox Christianity celebrates this feast annually on September 21.
Zero Emissions Day (also ZeDay), is annually observed by people in many countries around the world on September 21. The main message of this event is “Giving our planet a day off a year”.
Number of organizations supporting people with dementia initiated observation of World Alzheimer's Month, that is held every year in September. The most important events are held on September 21, that is World Alzheimer's Day.
Brazil, probably, is the only one country in the world that can boast two Arbor Days. The observances take place in September and March. The main events of Arbor Day (Dia da Árvore) take place on September 21, while due to climate characteristics this day is also observed during the last week in March in some of the Brazilian states.
The spring equinox falls in the Southern hemisphere on September 22 or 23. This day is known as the beginning of the spring there. However, Argentina, Uruguay and Bolivia celebrate Spring Day on September 21, one or two days before the actual equinox.
International Day of Peace is one of the observances, established by the UN General Assembly. This international day is observed annually on September 21.