Just remember, this is not some kind of Sekrit Pagan Language (TM); many of these words are in use in Europe today by pagans and non-pagans alike to denote these days. And yes, this shows a European bias, but then so do the commonly-used names for Wiccan holy days. These seem to be the names most frequently asked about in alt.pagan.
Samhain (31 Oct) -- Irish Gaelic for "summer's end." The standard Irish pronunciation is "sow-in" with the "ow" like in "cow." Other pronunciations that follow with the many Gaelic dialects include "sow-een" "shahvin" "sowin" (with "ow" like in "glow"). The Scots Gaelic spelling is "Samhuin" or "Samhuinn." There is no linguistic foundation for saying this word "samhane" the way it might look if it were English. When in doubt, just say "Hallows" or even "Hallowe'en."
Yule (@21 Dec) -- Norse for "wheel." It's pretty much pronounced just like it looks, although if you want to make a stab at a Scandinavian sound, it'll be more like "yool" and less like "yewl." This is the winter solstice.
Imbolg/Imbolc (1 Feb) -- Irish Gaelic for "in the belly." Pronounce this one "IM-bullug" or "IM-bulk" with a guttural "k" on the end. Other names include Candlemas; Brighid (pronounced "breed"), who is the Irish goddess whose festival this is; and Oimelc (pronounced EE-mulk), which means "ewe's milk" in Scots Gaelic.
Ostara (@21 Mar) -- Saxon name for a maiden goddess of spring, loosely connected to Astarte and Ishtar. This one's easy -- "o-STAHR-uh." Other names include Eostre (say "OHS-truh" or "EST-truh"). This is the spring equinox.
Beltane/Bealtaine (30 April) -- Irish Gaelic for either "fires of Bel" or "bright fires." If you want to try it in Gaelic, you can say "bee-YAWL-tinnuh" or "BELL-tinnuh." Unlike Samhain, this word can within the linguistic structure of its language of origin be pronounced like it looks -- "BELL-tane" -- without totally abandoning its original construction. Other names are Walpurgisnacht (vawl-PUR-gis-nahkt) and May Day.
Litha (@21 Jun) -- Norse or Anglo-Saxon for "longest day." You can say this one just like it looks, or you can try for a Scandinavian sound and say "leetha" with the "th" more like a "t." This is the summer solstice.
Lughnasadh/Lunasa or Lammas (1 Aug) -- The first is Irish Gaelic for "festival of Lugh" (a major Irish deity); the second is Anglo-Saxon for "festival of the loaves" ("hlaf-mass"). Don't panic at that spelling; the second (which is modern Irish as opposed to old Irish) tells you all you need to know. Say "LOO-nah-sah." (Some people maintain that the Scots dialect says it "LOO-nah-soo.") Lammas is just like it looks, "LAH-mus." Mabon (@21 Sep) -- This is believed to be a form of the Welsh word for "son." Therefore, it would probably be pronounced "MA-bon" with the "a" like in "mass." However, most Wiccans and pagans say "MAY-bon." This is the autumn equinox.