Source 2: http://www.druidsgrove.com
There was a series of fire-festivals, occurring at approximately 12-week intervals, and spaced between the seasonal festivals of solstices and equinox (thus, a festival roughly every six weeks.) These fire-festivals would last three days, beginning at sunset on the first day, and would be the best time for sacrifices and divinations. They are:
Samhain (1st November: pronounced SOW-win) The word literally means "end of summer". Traditionally, it is the Feast of the Dead, and beginning of the new year. Death came before Birth in the Druidic cycle of life, because before new growth can occur, there must be room for it. On this day it was thought that the boundary between this world and the Otherworld is weakest, and so passage between the worlds is smoother, and as they might be listening a little closer it is a time to remember and respect all those who died during the year. Games, feasts, and bonfires were held in honor of the dead, and often the Faeries would hold revels of their own, and invite mortals to join them. At Samhain, every fire in Ireland was extinguished and re-kindled from the "need fires" that were lit at the ritual centers of Uisneach and Tara, distributed by runners with torches.
Imbolc (1st February: pronounced IM-volk) The Return of Light. The ewes begin lactating around this time of year, and it is a sign that winter is coming to an end. In the British Isles spring flowers are already blooming at this time of year. Perhaps divinations were cast to determine when spring would come (from this practice we might have got Groundhog Day.) Imbolc celebrates the coming springtime and preparations for the planting season are begun. In Anglo-Saxon and Wiccan culture, Imbolc is sometimes called Candlemas. Imbolc was sacred to the Goddess Brigid, and the rituals on this day tended to center upon the home and hearth.
Beltaine (1st May: pronounced BEL-tain-yuh) The Fires of Bel. Spring has arrived, and the people give thanks. This was a day of fertility and life, often the choice day for marriages. This is the beginning of the summer half of the year, and the mid point of the seasonal cycle. Fairs, dances, and divination games to determine the identity of future marriage partners were held at this time of year, and often there would be a minor baby boom nine months later…
Lughnasad (1st August: pronounced LOO-na-shav) The Feast of Lugh. The essential harvest festival, to give thanks to the Earth for Her bounty. The name is a reference to the Irish god Lugh of the Long Hand, son of the Sun, who defeated Balor in the Battle of Magh Tureadh and won the knowledge of animal husbandry for His people on this day. Lugh is said to have instituted funeral games for his foster-mother Taltiu who died in the battle against Balor; accordingly, Lughnasad festivals in Celtic times were characterized by athletic competitions. In Anglo -Saxon and Wiccan culture, this festival is called Lammas, or "loaf-mass", as it celebrates the end of last year's harvest and the beginning of the current harvest.
I understand that Australians, and other residents of countries in the southern hemisphere who celebrate these festivals, do it in reverse order, because these dates are for northern-hemisphere seasons. It would make sense for them to celebrate Beltaine on 1st November, for example.
In Wales, there was an annual festival called the Eisteddfod, which was a bardic musical and poetry competition. It still exists, alternating between North and South Wales. It is against the rules of the modern Eisteddfod to speak any language but Welsh on the performance stages!
During these ancient festivals, great bonfires were built on hilltops and kept burning throughout the whole of the fire festivals. By day, there would be carnival-like celebrations, and by night, serious rituals. Cattle were driven between bonfires to purify them, and couples would run and leap over the flames, often completely naked, also for purification (and it was fun!) Some sites were centers for the "perpetual chant", where Druids in rotation would chant incantations without stop; during festivals the entire community would join the chant.
Astronomical celebrations (the solstices and equinox) have only passing reference in the source literature (that is, the myths, Caesar, etc.), and so would appear to have less importance in the Celtic cosmology, but astronomical alignments are found everywhere in the archaeology, particularly in the archaeology of the Neolithic pre-Celtic culture. There are hundreds of stone circles, round barrows, menhirs, etc. with solar, lunar, and stellar alignments.