Agathos Daimon means “good spirit” and is a religious observance held on the second day of each lunar month, immediately following the Noumenia. It is the third celebration of a trio of household monthly observances. A good spirit usually refers to a type of divine being that is less powerful than a God, is personal to each family, and can bring the family good luck, protection, or some type of assistance. Household spirits are usually seen as either snakes or a s a young man with a horn of plenty in hand.
I celebrate the Agathos Diamon by pouring a libation to the spirit and asking for his continued blessings on our family. If there is something in particular that our family wishes help with, I may give an additional offering to our family's protective spirit. Although I know I can always approach the Gods directly, the Agathoi Diamones are seen to be helpful intermediaries between the Gods and man.
The second day of every Athenian month was also a sacred day, devoted to the Agathos Daimon (good spirit). The name “daimon” does not mean the evil demon of modern Christianity, (although it did have a negative form, called the kakodaimon), but was thought to be an aspect of Zeus, as Zeus Ktesios, Charitodotes, and Epikarpios, titles as giver of increase and joy. Agathos Daimon is most often represented in the form of a snake, a symbol of healing. However the daimon is also a function of one’s being, a characteristic inherently neither good nor bad. Hence, one prays for a good daimon, an eudaimon, and goodness from the gods for the coming month and also for the favor of father Zeus as Agathos Daimon. Burkert (Greek Religion, p. 181) says that “One must be on good terms with it.” And Pindar sang that "The daimon active about me I will always consciously put to rights with me by cultivating him according to my means" (Pyth. 3.108f) and "The great mind of Zeus steers the daimon of the men whom he loves" (Pyth. 5.122f). The philosopher Sokratēs talks of his own daimon as a small voice which speaks to him and warns him to refrain from certain actions (Plato, Apology, 31d).
Note: the above is an expansion of an article written by Bob Clark.