We’re all familiar with the major astronomical milestones in the year—the summer and winter solstices, when our hemisphere is tipped maximally toward or away from the sun, and the spring and fall equinoxes, when each day worldwide has roughly 12 hours of sunlight and 12 of darkness. These dates separate the seasons—the spring equinox marks the beginning of spring, and so on. They are to the calendar what north, south, east, and west are to the compass.
In the same way that the angles between the four cardinal compass points are divided by four ordinal points (northeast, southeast, and so on), the seasons defined by the four astronomical dates are divided by four cross-quarter days. These were Gaelic festivals in medieval times. They are Imbolc (February 2), Beltane (May 1), Lughnasadh (August 1), and Samhain (October 31). Imbolc (pronounced i-molk’) lines up with our Groundhog Day.
Most of us are familiar with the idea that on Halloween (All Hallows’ Eve, or Gaelic Samhain), spirits from the next world could enter ours, which is why ghosts and the dead are associated with Halloween. In Gaelic mythology, the veil between our world and the next became thinner not only on Samhain but for each of the cross-quarter days. These days provided opportunities for divining the future using information from beyond.
In the same way that Christmas subsumed pagan holidays on the winter solstice like Saturnalia and Yule, the Christian holiday of Candlemas subsumed Imbolc (February 2). Candlemas celebrates the presentation of Jesus at the Temple in Jerusalem 40 days after his birth.
The Celtic goddess Brigid (or Brigit) was associated with Imbolc, but she too was subsumed into Christianity as Saint Brigid.
Both pagans and the Christians who followed them observed nature on Imbolc/Candlemas to glean clues to how much longer winter would last. Would it go the full six-and-a-half weeks until the spring equinox or would it be a more gentle winter?
German immigrants to America had used hedgehogs to help predict the weather. If it was sunny and the hedgehog could see its shadow, winter would go the distance. But if it was cloudy, winter would be shorter. With no hedgehogs in America, they switched to groundhogs. (The two animals are not closely related, but their habitats are similar.)
This Imbolc, whether you follow Punxsutawney Phil (the center of the biggest Groundhog Day celebration in Punxsutawney, Pennsylvania) or some lesser-known groundhog prognosticator, keep in mind the spiritual origins of the tradition.

That’s where the truth lies, right down here in the gut.
Do you know you have more nerve endings in your gut
than you have in your head?
You can look it up.
I know some of you are going to say,
“I did look it up, and that’s not true.”
That’s because you looked it up in a book.
Next time, look it up in your gut.
I did. My gut tells me that’s how our nervous system works.
— Stephen Colbert

(This is a modified version of a post originally posted 2/1/12)
Photo credit: wvholst