Before the development of calendars and clocks, our ancestors relied on the cycles of the stars, sun and moon to tell the time of day and the time of year. From the day and night sky, our ancestors gained a profound sense of cyclical time, of order and renewal in the predictability of nature. Today, we their descendants have also inherited many of their rituals rooted in cyclical time.
One of the oldest cycles our ancestors used to measure the passage of days and mark special rituals and holidays was the reliable phases of the moon. Historically, Easter occurs on the first Sunday following the first full moon that occurs on or after the day of the vernal equinox. This year, the first full moon after the equinox is March 27. Thus, Easter this year is celebrated on Sunday, March 31. The first night of the Jewish Passover is also connected to the first full moon of spring and observance begins tonight with the first Seder.