In the course of doing yet more research for this lunar observance thing, I’ve been reading through the relevant sections of the Yajurveda. I was initially thinking of basing some prayers for the occasion on the Vedic ones for the paurṇamāsa yajña, but my god have you seen them? Perhaps not.
The Black Yajurveda also recounts a myth explaining why Soma and Agni are invoked at the full moon sacrifice as well as Indra, the reason being that they helped him in defeating the demon Vṛtra. The lead-up to this also gives the story of how Vṛtra came to be, in revenge for Indra’s killing of Tvaṣṭṛ’s son Viśvarūpa. Viśvarūpa was the purohita of the gods, according to the Black Yajurveda (he’s not so named in the Rigveda- here viśvarūpa only occurs as an epithet of Tvaṣṭṛ himself), and thereby a brahman. So a little story about how Indra cleansed himself of the guilt of brahminicide is told, which I thought would work nicely in a Celtic context:
After travelling as far accross the world as the sun sees, Gobannos son of Taranis and Danu took as his wife Derga the Red, a woman of the Ancenetlī. The son of Gobannos and Derga was Ollodelvos, so named because of his three heads: one which would take nothing but mead, one which would take nothing but milk and one which would take nothing but wine.
Nourished on milk, mead and wine the boy grew in strength and wisdom. He learnt the charms and the curses, and the sacred hymns which are the life of the Gods. He learnt to recite the twice nine sacrificial formulas, each one a benefit to the righteous man. And so, when he reached the fullness of his manhood, Ollodelvos son of Gobannos was named by the Gods as their gutuatir, charged with pouring out the libations on their behalf- for even the Gods offer sacrifice in order to maintain the truth of the world.
But Ollodelvos was found to be faithless. For every libation he poured on behalf of the Gods, he poured out another in secret for his mother’s people, the Ancenetlī. And so Lugus, to whom the punishment of faithless acts falls, took his spear and slew Ollodelvos, smiting off each of his heads in turn, snakes falling from each of his three necks.
But the creatures of the earth recoiled in horror, for Lugus had slain a druid, and a kinsman at that. For a year Lugus stayed his guilty hand, and the Gods suffered greatly as a result: the faithless went unpunished, warriors went to battle without courage. So after a year, Lugus went to the earth and said to her: “Take one third of my guilt!”
“I shall if you grant me a boon,” said the earth. “The ploughing of men will kill me, let me not be so killed.”
“I grant you this boon,” replied Lugus, “the ploughing of men will give you increase within a year, it will not kill you.” And so it is: before a year has passed, a ploughed field yields up bounteous crops. The earth took one third of Lugus’ guilt into herself, and this became the cracks in the rocks of the earth: for this reason you should not sacrifice with a fire built on a crack, for you will sacrifice to his guilt.
Then Lugus went into the forest and said to the trees: “Take one third of my guilt!”
“We shall if you grant us a boon,” said the trees. “The pruning of men kills us, let us not be so killed.”
“I grant you this boon,” replied Lugus, “the pruning of men will give you increase within a year, it will not kill you.” And so it is: before a year has passed, new shoots are seen on a pruned tree. The trees took one third of Lugus’ guilt into themselves, and this became the red sap of the trees: for this reason you should not drink the red sap of a tree, for you will drink of his guilt.
Then Lugus went to the women and said: “Take one third of my guilt!”
“We shall if you grant us a boon,” said the women. “The fucking of men kills us, let us not be so killed.”
“I grant you this boon,” replied Lugus, “the fucking of men will give you increase within a year, it will not kill you.” And so it is: before a year has passed, a woman who has had intercourse with a man will bear a child. The women took one third of Lugus’ guilt into themselves, and this became the blood of their courses: for this reason you should not offer menstrual blood to the gods, for you will offer them guilt.
So cleansed of his guilt, Lugus took up again his spear and went to rejoin the gods.