The Celtic year was divided into two halves, the dark and the light. Samhain was the beginning of the dark half, with its counterpart, Beltane beginning the light half. Between these two 'doors' or portals fell Imbolc, on February 1, and Lughnasadh or Lammas, celebrated on August 1, quartering the Celtic year. These quarters were again divided by the solstices and equinoxes, which were known as the four Albans.
Refer to: http://www.sacredfire.net/festivals.html