For us a day begins at sunrise. But for the ancient Celts it began at twilight. A festival would begin at twilight and end at the next twilight. For example Samhain (which we call Halloween) would begin at twilight on October 1 and run until twilight on November 1.
Thus dark was the beginning rather than light. In the same way, the new year begins in the dark half of the year (winter.)
This concept is common to many cultures, including Judaism. Every wonder why? Perhaps it's because of this:
In the beginning God created the heavens and the earth.
Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.
And God said, "Let there be light," and there was light. God saw that the light was good, and He separated the light from the darkness. God called the light "day," and the darkness he called "night." And there was evening, and there was morning—the first day. ~Genesis 1:1-5 NIV
Note "there was evening and there was morning--the first day."