The year is good when Christmas comes during the first phase of the moon.
Note: The first phase of the moon is the new moon. A new moon occurs about every 29 1/2 days, the time it takes for the moon to orbit the earth. More specifically, it occurs precisely when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0 degrees. In other words, it occurs when the moon is exactly between the sun and the earth. At this point no light is reflected off the moon’s surface to the earth. Therefore, when you look at a new moon in a clear night sky, you see no moon, or a completely shaded moon. Ancient Celts knew this precise moment.
A new moon had cosmic significance for the Gael. For the ancient Druids, it is believed that the moon and the planets represented Celtic gods. For this reason, there was a geis, a proscription against speaking or writing their names. Anyone who violated a geis was cursed with an evil spell. So the moon and the planets were referred to with euphemisms. That is why the Irish word for the moon is ‘gealach’ (‘Gealaí’ is the genitive singular form.), a euphemism that literally means ‘brightness.’
When Christianity supplanted Druidism, the new moon took on a new cosmic significance. Jesus Christ was born under a new moon. The beginning of the Savior’s life corresponded with the beginning of the life of the moon. So it is obvious that it is good luck whenever Christmas corresponds with the new moon. The next time the new moon occurs on Christmas will be December 25, 2000 at exactly 5:22 P.M. GMT. However, the good year will mostly be in 2001 since the Irish year begins in November and ends in October.