Midwinter: The Birth of the Sun
Midwinter marks the shortest day of the year, and marks the darkest, coldest part of winter, when the sun appears to be at its weakest. The decline of the sun is often accompanied by careful vigils and the lighting of fires and candles to encourage the return of the sun's strength.
Christmas, of course, predates European Christianity. Curiously, however, it shares a similar origin to later festivals, as it was quite deliberately instituted to compete with pagan solar nativities celebrated in Rome during the earliest years of Christianity. The first versions of the Christian observance of the birth of Christ were offered as an alternative celebration to the boisterous ruckus of the Saturnalia. Many of the customs we associate with Christmas in fact originated with the Saturnalia celebrations, and European Christians often shunned the holiday as a pagan remnant. The Celtic (and Norse) pagan contributions to the holiday include mistletoe and even Christmas trees.