Sanhamitta Day honors the Arahant Ven. Sanghamitta (daughter of King Ashoka) for her travels to Sri Lanka to spread the Dhamma there and set-up the Bhikkhuni Order in Sri Lanka. It occurs on the Full Moon day of December. She came to Sri Lanka with a sapling of the Bodhi Tree, the same tree that the Buddha attained enlightenment under. It is the oldest living tree with a known planting date (288 BCE). See: Jaya Sri Maha Bodhi