Imbolc, the pagan festival of purification, is celebrated on February 1. One of the "cross-quarter” celebrations, Imbolc does not correpond to a seasonal or solar event such as an equinox or solstice, but instead lies halfway between those points. As such, it joins Beltane, Lammas (Lughnasadh) and Samhain to form the eight great festivals of the year.
So how and why is Imbolc celebrated? In colder climates, Imbolc is the first sign of spring—lambs are born around this time, into what is often a very chilly atmosphere. In agriculture, fields are prepared for planting after the frosts are gone. Because of this theme of preparation, spring cleaning often starts around Imbolc.
Early Christianity and paganism were part and parcel of the same thing. Nowhere is this more plain than in Imbolc traditions. While Christianity sought to integrate paganism by renaming gods and traditions to saints and feasts, those Christian roots still show up in many of the Imbolc rituals.