Samhain - 1888, from Ir. samhain (Gaelic Samhuinn), from O.Ir. samain, lit. "summer's end," from O.Ir. sam "summer" (see summer) + fuin "end." Nov. 1, the Celtic festival of the start of winter and of the new year.
Yule - O.E. geol, geola "Christmas Day, Christmastide," from O.N. jol (pl.), a heathen feast, later taken over by Christianity, of unknown origin. The O.E. (Anglian) cognate giuli was the Anglo-Saxons' name for a two-month midwinter season corresponding to Roman December and January, a time of important feasts but not itself a festival. After conversion to Christianity it narrowed to mean "the 12-day feast of the Nativity" (which began Dec. 25), but was replaced by Christmas by 11c., except in the northeast (areas of Danish settlement), where it remained the usual word. Revived 19c. by writers to mean "the Christmas of 'Merrie England.' " First direct reference to the Yule log is 17c. O.N. jol seems to have been borrowed in O.Fr. as jolif, hence Mod.Fr. joli "pretty, nice," originally "festive"
Imbolc (Candlemass) - O.E. candelm?sse (from candle + mass (2)), feast of the purification of the Virgin Mary (Feb. 2), celebrated with many candles, corresponding to Celtic pagan Imbolc. The name Imbolc comes from the Old Irish i mbolg, 'in the belly', apparently in reference to either pregnant ewes or milking. The oldest etymology, that of the ninth century Cormac's Glossary, derives imbolc (also oimelc) from 'the time the sheep's milk comes'.
Ostara (Eostre) - O.E. Easterd?g, from Eastre (Northumbrian Eostre), from P.Gmc. *Austron, a goddess of fertility and spring, probably originally of sunrise whose feast was celebrated at the spring equinox, from *austra-, from PIE *aus- "to shine" (especially of the dawn). Bede says Anglo-Saxon Christians adopted her name and many of the celebratory practices for their Mass of Christ's resurrection. Ultimately related to east. Almost all neighboring languages use a variant of Latin Pasche to name this holiday. Easter egg attested by 1825, earlier pace egg (1610s). Easter bunny attested by 1909.
Beltane - early 15c., from Lowland Scottish, from Gaelic bealltainn "May 1," important Celtic religious rite marking the start of summer, probably lit. "blazing fire," from PIE base *bhel- (1) "to shine, flash, burn" (see bleach) + O.Ir. ten "fire," from PIE *tepnos, related to L. tepidus "warm." But this derivation of the second element is hotly disputed by some on philological grounds, and fires were equally important in the other Celtic holidays. Also known as "Old May Day," because after the 1752 calendar reform it continued to be reckoned according to Old Style; it was one of the quarter-days of ancient Scotland.
Litha (Midsummer) - O.E. midsumor, from mid "mid" + sumor "summer." Midsummer Day, as an English quarter-day, was June 24. Astronomically June 21, but traditionally reckoned in Europe on the night of June 23-24. (could not find an origin for the term Litha readily. I'll have to do a bit more research on that one.)
Lammas - Aug. 1 harvest festival with consecration of loaves," O.E. hlafm?sse, lit. "loaf mass," from hlaf + m?sse.
Lughnassadh - From Old Irish Lugnasad (?Lugh's festival?).
Mabon - The name Mabon is derived from the Common Brythonic and Gaulish deity Maponos. Similarly, Modron is derived from Common Brythonic and Gaulish deity Matrona. The language changes creating the Middle Welsh form are:
* dropping of masculine singular -os and feminine singular -a endings
* p > b
* a > o
* t > d
These changes are discussed in Sims-Williams (2003).
The name Mabon has special connections to Hadrian's Wall where a cult of Apollo Maponos was practised by the Roman soldiers based there.
http://www.etymonline.com/index.php
http://en.wikipedia.org/wiki/Mabon_ap_Modron
Chadwick, Nora, The Celts, 1970:181.
Chormaic, Sanas, Cormac's Glossary, trans. John O'Donovan, 1868.
Hamp, E. P., 'imbolc, oimelc', Studia Celtica, 14/15 (1979/80) 106.
MacBain, Alexander, An Etymological Dictionary of the Gaelic Language, 1911.
O Cathain, Seamus, 'The Festival of Brigit the Holy Woman', Celtica, 23 (1999) 231-260.
Ruickbie, Leo, Open Source Wicca: The Gardnerian Tradition, 2007.