Imbolc was a three day holiday beginning on the 31st of January and ending on February 2nd. This was a holiday sacred to the Goddess Brighid, a time of renewal and rebirth. Brighid is an ancient Celtic triple Goddess (she has two sisters also named Brighid), and a daughter of the Dagda. The Dagda was the Father God of Ireland. Imbolc is representative of many things; the quickening of new life, the celebration of fire and water, and lactation of ewes. Fire was celebrated with the lighting of candles to symbolize the growing light of the sun and the lengthening of days. Water was recognized in the celebration of wells which were another symbol of Brighid. They were adorned with greenery to symbolize the coming of spring. Celebrants would circle the wells clockwise or deosil and drink it's water to bring about good fortune.
Imbolc is the midpoint between Winter and Spring. Weather divination was as popular at Imbolc as it is today. It was said that Cailleach would begin to lose her hold on winter, she would go out and look for materials to stoke her fire, if she found dry twigs to use for kindling she would extend winter, but if it was rainy and kindling was damp, winter would end. Most likely this is how the modern day groundhog tradition was started, after all he's only going to see his shadow if it's sunny out.
This holiday is also known as Oimelg which means "ewe's milk". This was an important holiday for ancient pagans as a lactating ewe meant lambs in the spring and milk, butter, and cheese to hold them for the rest of the Winter. New life is an important aspect of Imbolc which translates into "in the belly". This is probably referring to the quickening of life in the womb, specifically that of the calves to be born at springtime. Imbolc was often celebrated with lambs meat.
Many of the activities from Imbolc of yore are still popular today. Doors are left open to invite Brighid to enter (and also possibly some fresh air after a long stuffy winter), and at night ashes can be spread outside and if there are footprints in the morning it is a good omen that Brighid has visited. Brighid's beds, also called God's eyes, are made and hung from doors for protection, and strips of cloth or ribbons, known as Brighid's cloak, are hung from trees to catch the mornings dew. These can be used to attach to your clothes as a charm for healing. While there is no longer the fear of imminent death due to harsh climates Wiccans today still celebrate Imbolc with the hope of new beginnings.