While the light half of the Wheel of the Year is about growth and outward activity, the dark half has a more inward focus, both drawing into the self, and back into the home during the cold winter months.
Mabon, the autumn equinox (around September 21st). The first day of autumn, and the first day of the dark half of the year. This is the sunset of the year, it is a time of balance, of (nearly) equal light and dark - but after this time, the nights will be longer than the days. Mabon is the second of the three harvest festivals. It is a time of celebration, of giving thanks for the bounty of the light half of the year.
Samhain (November 1st, although celebrations usually begin the evening before), the sun has set, the sky is dark - but it's not quite midnight yet. This is the peak of the autumn season. This is the final of the three harvest festivals. A time to hurry and bring in any last crops from the fields before winter sets in. It is also the harvest of animals, a time to cull herds, and a time to hunt. Many of us likely don't have physical herds that need culling, but we can still remove unnecessary things from our lives at this time.
At Beltane the energies of spring and life were at their peak, and so at Samhain the energies of autumn and death are now at their height. The barrier between the worlds is again at it's thinnest point, but this time we honor those who've gone before us - the ancestors.
Yule, the winter solstice (around December 21st). The longest night of the year, but a night filled with the promise of the return of the light. The wheel has turned to midnight, and as a new day starts at midnight, so the new year comes with Yule - the sun and the year both being reborn. Yule is the biggest holiday of the wheel for me. It begins with the eve before the solstice, and is celebrated for 12 days. During the longest night a candle is kept lit, keeping the spark of light alive through the darkness, until the sun is reborn in the sky. The celebration lasts through Christmas and the calender new year, and so there are several opportunities to gather with friends and family, and celebrate the promise of returning light on cold winter nights.
Imbolc (February 2nd), winter's mid-point. We've passed through the bulk of the night, we known dawn is coming soon, but it's still a ways off. This is a time to prepare for the coming light half of the year. It is a quiet time, a time for purification, for tidying up and readying ourselves both physically and spiritually for the active summer months ahead. Candles are lit in honor of the slowly returning light. In New Hampshire we are still in the dead of winter during this holiday, but even so, the first faint signs of the coming spring can be seen... days lasting just a little longer, and soon we see winds not quite as bitter. There's still a wait before we really feel the warmth, but we know it will come eventually.
Then the Wheel turns back to Ostara, the light half returns, and the cycle continues.