Imbolc (Imbolg) the festival marking the beginning of spring has been celebrated since ancient times. It is a Cross Quarter Day, midpoint between the Winter Solstice and the Spring Equinox, it can fall between the 2nd & 7th of February when calculated as the mid point between the astronomical Winter Solstice and the astronomical Spring Equinox, in 2019 it falls on February 4th. The astronomically derived date is later than the traditional date of January 31st / February 1st.
Imbolc derives from the Old Irish i mbolg meaning in the belly, a time when sheep began to lactate and their udders filled and the grass began to grow.