Mėnuo (Latvian Mēness, Old Prussian Menulis) is the Baltic moon, a male deity and the moon itself, whose waxing and waning the dainos explain through the celestial wedding: he marries Saulė, is unfaithful with the morning star (Aušrinė or Auseklis), and Perkūnas cuts him with a sword, which is why the moon wanes. He is one of the Dievo sūneliai, a son of Dievas, and a measurer of time and seasons rather than a ruler of the dead. The evidence is the daina corpus, not a cult archive, and the story exists in several regional variants.
Modern Practice
Mėnuo is kept by counting: the new moon named, the full moon marked, silver and milk left out, and no oath sworn on a waning moon. The practice is timekeeping with attention - the calendar read off the sky rather than off a page.