In ancient times, the Celtic New Year was celebrated with the Samhain festival, a holiday marking the beginning of winter in the Northern Hemisphere around October 31st. This was accurately described as the time when the light half of the year (summer, when the days are longer and warmer) gave into the dark half (winter, when the days are shorter and colder). This time also coincided with the midnight culmination of a star cluster we know in astronomy as the Pleiades. According to the Celts, the Pleiades were the window to the other world. They believed that when the Pleiades reached their highest point in the sky at midnight, that the barriers between worlds became fluid and the spirits of the dead were able to cross into the world of the living. Today we carry on the tradition with the festival of Halloween, often dressing up as ghosts or other-worldly creatures.
The Pleiades are more commonly known in New Zealand as Matariki, but were known to the Celts by a myriad of names: Crannarain, Grica, Griglean, Grioglacha, Meanmach, and more recently by the Irish, in Gaelic as seachd-reultan (the seven stars) or Criathar (the sieve). We celebrate the festival of Matariki here in New Zealand in June during the time of year that the Pleiades rise above the horizon just before sunrise. By October, the Pleiades rise much earlier, and we can see them high in the sky in the middle of the night, just like the ancient Celts.
You might be interested to know that our modern calendar is actually based off ancient solar calendars, and many of our festivities are linked to astronomy because of this. Think of our year as a circle, and split this circle into quarters for each of the seasons. Half way through each season we have a Quarter Day which marks the summer solstice, autumn equinox, winter solstice or spring equinox.
A solstice occurs twice a year and is when the Sun is at its highest or lowest point in the sky. An equinox marks when the Sun is midway between solstices and the length of day and night are the same. Currently, we mark the first day of every season on one of these Quarter Days, e.g. the beginning of our winter is on our winter solstice. However, the seasonal changes centre on the solstices and equinoxes. So it is actually the Cross-Quarter Days that mark the transitions between seasons and Halloween is one of those Cross-Quarter holidays, halfway between the Northern Hemisphere’s autumn equinox and winter solstice.