The wisdom and the comfort of the Winter Solstice is that it's nature's own reminder of the intimacy of light and dark -- like the front and back foot in walking, says Sekito's poem. We'd find it silly to believe that only the front foot makes progress, especially since the propulsion and momentum come from the back foot. Just so, our lives don't move at all unless light and dark are both present, waxing and waning to their own rhythm regardless of our calendared highs and lows.
Zen Master Dōgēn said not to call winter the beginning of spring. But maybe he was wrong. For as much as winter's long night connotes death and decay, it also invites rest and surrender to the nourishment of the dark, rich earth -- without which spring's exuberant creativity couldn't happen at all.