The Noumenia was the first day of the lunar month and also a religious observance in ancient Athens and much of Greece. The Noumenia was marked when the first sliver of moon was visible and was held in honor of Selene, Apollon Noumenios, Hestia and the other Hellenic household Gods. The Noumenia was also the second day in a three day household celebration held each lunar month; Hekate's Deipnon is on the last day before the first slice of visible moon and is the last day in a lunar month, then the Noumenia which marks the first day in a lunar month, followed by the Agathos Daimon (Good Spirits) on the second day of the Lunar month.
As celebrated in Athens 
The Noumenia was considered, in the words of Plutarch, "the holiest of days" and no other religious festivals were allowed on this day, nor were any governmental meetings scheduled to take place on the Noumenia. It was a day of relaxation and feasting for Athenian citizens, at home, or for males, with religious fraternities. Wrestling matches also took place at the palaestra.
It was celebrated by a public ritual on the Acropolis and by private offerings of frankincense, flower garlands, wine, and barley cakes placed on home altars and household shrines to Hekate and Hermes, which had been freshly cleaned the day before as part of the Deipnon.
The official state rituals for this day included small offerings to gods and goddesses seen as protectors of Athens, such as Athena Polias and Poseidon, but the most important was made to the guardian snake of the city. The snake was kept at the Erechtheum, a temple housing the cult statue of Athena Polias, the mark of Poseidon Erechtheus' Trident and the salt spring, the sacred olive tree that sprouted when Athena struck the rock with her spear, and the burial places of Athens first two kings, Cecrops and Erechtheus. The snake was believed to be the living spirit of King Cecrops.
Other Greek cities 
Modern Practices 
Those reviving the indigenous religions of Greece perform many of the same household religious practices and hold the Noumenia as important as their ancient counterparts. They make offerings such as incense or honey cakes to Selene, Apollon, Hestia, and their household Gods at their family altar. Other ways they mark the day include decorating their home or home altar with fresh flowers, eating a special family meal, making honey cakes, and replacing the ingredients in the kathiskos with fresh water, oil, and fruit, and bits of food. The Noumenia also considered an auspicious day to begin new projects.
- Illinois Classical Studies 19, Article Hereclitus on Old and New Months by David Sider, page 11-13
- The Harvard Theological Review, Vol. 65, No. 2, Apr., 1972 article titled The Noumenia and Epimenia, page 291
- Handbook to Life in Ancient Greece, by Roy A. Adkins and Lesley Adkins, page 351, published by Facts on File; Updated edition (January 2005) Also, Porphyry, On Abstinence from Animal Food, Book 2, section 16, translated by Gillian Clark, published by Duckworth, 2000
- The Harvard Theological Review, Vol. 65, No. 2, Apr., 1972 article titled The Noumenia and Epimenia, page 294-296
- Carol Strickland (11 April 2001). The Annotated Arch: A Crash Course in the History of Architecture. Andrews McMeel Publishing. pp. 15–16. ISBN 978-0-7407-1024-7. Retrieved 24 July 2012.
- Herodotus, 6.57, translation by G. C. Macaulay, published by Macmillan, 1890
- Author Sannion (H. Jeremiah Lewis), guest writing for The Wild Hunt, about the Noumenia. "The Noumenia is perhaps just as popular and universal for contemporary Hellenic and Greco-Egyptian polytheists as it was for our cultural ancestors. Ours is a diverse community, and though we may have different festival calendars, honor different gods, and even employ different methods of worship – most of us still do at least something to mark the Noumenia." http://wildhunt.org/2009/07/kala-noumenia.html
- Article on Noumenia on Temenos, maintained by members of Hellenion, a 501c3 religious organization "dedicated to the revival and practice of Hellenic polytheism. We approach Hellenic religion from the reconstructionist perspective, which includes both an emphasis on historical precedent and respect for personal spiritual inspiration." https://sites.google.com/site/hellenionstemenos/Home/festivals/noumenia