The Celtic calendar was lunar based, with thirteen months. Extra days as needed were added at new year's as a "time between times." The ancient Celts divided year into a wheel of eight segments, each with a corresponding festival. The four fire festivals, so-called because all include bonfires as part of celebration, take place on last evening of a month and following day. The Celts, like Jews, count a day from sunset to sunset. That's why we celebrate All Hallow's Eve, Midsummer's Eve, and so on.
These four fire festivals are tied to agricultural cycle as follows:
Samhain is celebrated on October 31-November 1 (our Halloween). It is end of harvest, beginning of winter and once marked Celtic new year. At Samhain, barrier between our world and Otherworld thins, allowing contacts between spirits (faeries) and humans. Normal rules of human conduct do not apply and one may "run wild". This was also a festival of dead and church was easily able to transform these holidays into All Saint's Day (November 1) and All Soul's Day (November 2).
Imbolc is celebrated February 1-2 (later transformed into Candlemas by church, and popular now as Groundhog Day). Imbolc marked beginning of Spring, beginning of new life (in Britain beginning of lambing season). Dedicated to ancient mother goddess in her maiden aspect, it was later transformed into a feast day for Irish saint of same name (and attributes), St. Brigid.
The third festival of agricultural year is Beltane (Bealtunn in Scots Gaelic, meaning May Day), celebrated April 30-May 1. The god, Bel (or Cernunnos, horned god of Ireland) dies but is reborn as goddess' son. He then impregnates her ensuring neverending cycle of rebirth. This is very basic fertility worship. May Day traditions includes young people picking flowers in woods (and spending night there), and dance around May Pole, weaving red (for god) and white (for goddess) streamers round and round. A great bonfire celebrates return of sun.
The final celebration of agricultural year is Lughnasadh (Lammas in England), feast of god Lugh and first fruits of harvest (generally wheat or corn). Lughnasadh is celebrated August 31-September 1. At Lammas, Corn King dies (to be reborn at spring), ensuring plenty for winter.