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In any lunar calendar, however, rituals and holidays linked to the phases of the Moon slip through the seasons unless the lunar months are somehow anchored to the solar year. Realizing that the seasonal, or solar, year doesn't exactly match a whole number of lunar months, people devised a variety of ways of adding an extra month now and then to bring the rhythms of the Sun and Moon back into synchronization.
After 8 solar years a lunar phase recurs on the same calendar date plus just 1 or 2 days, forming almost as accurate a prediction technique.

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