Beginning at sundown today, this is the Night of Forgiveness, or Lailat al-Bara'ah (or Shab-Barat). In Muslim belief, Allah fixes the events of the upcoming year on this night, including the fate of all persons, and so it is a night of seeking God's mercy and forgiveness, as well as committing sincerely to a sin-free life. A night-long vigil of prayers and the reading of the Qur'an are important aspects of the festival. Shi'ite Muslims also celebrate the birthday of the Twelfth Imam, who will return someday to redeem the world. http://imamsalim.com/blog/2015/05/31/
In Buddhism, especially Theravada, today is Vesak, the festival commemorating the birth, enlightenment, and death of Gautama Buddha. Vesak is observed on the full moon day of the month of Vaisakha (Hindu calendar), which is usually in May. Flowers and flags decorate temples, with Buddhists attending temples as monks chant the sutras.
(From the 2015 Interfaith Calendar of the Diversity Awareness Partnership of St. Louis---see dapstl.org for more information---and various online sources.)