From Halopedia, the Halo wiki
Rainfall was the name given to a season on the moon in an unknown galaxy to which Bornstellar-Makes-Eternal-Lasting and Chant-to-Green retired to live and raise their son. It was the third of five lunar cycles, or seasons, in the moon's standard year. As its name suggests, Rainfall was typically characterized by a period of heavy precipitation, which made for positive agricultural prospects for Bornstellar and his family, provided they had planted enough.
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