Sanghamitta Day
Sanghamitta Day is held on December 21. Theravada Buddhism. This event in the third decade of the month December is annual. 
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Saṅghamittā (Saṅghamitrā in Sanskrit) was the eldest daughter of Emperor Ashoka (304 BC-232 BC) and his first wife, Devi. Together with her brother Mahinda, she joined an order of Buddhist monks. Later, the two went to Sri Lanka to spread the Buddha's teachings as requested by King Devanampiya Tissa (250 BC-210 BC), Ashoka's contemporary. At first, Ashoka was reluctant to send his daughter on a foreign mission. She was sent to Sri Lanka along with several other nuns to be initiated into Buddhist monasticism of bhikkhunis at the request of King Tissa to ordain Queen Anulā and other women of Tissa's court in Anuradhapura who wished to be ordained as nuns after Mahindra converted them to Buddhism.
Similar holidays and events, festivals and interesting facts
Navy Day in Sri Lanka on December 9
Pancha Ganapati in Sri Lanka on December 21
Duruthu Full Moon Poya Day in Sri Lanka on January 14 (In honour of Lord Buddha's first visit to Sri Lanka);
Thai Pongal in Sri Lanka on January 15 (is a harvest festival dedicated to the Sun. It is a four-day festival which according to the Tamil calendar is usually celebrated from January 14 to January 17);
Sri Lanka Independence Day on February 4
Navam Full Moon Poya Day in Sri Lanka on February 12 (The Buddha proclaims for the first time a code of fundamental ethical precepts for the monks);
Magha Puja on February 12 (Full moon, 3rd Thai lunar month)