Loreena McKennitt’s “songs for the season” are haunting for their beauty and her fascinating voice. Blending traditional Irish music with worldbeat and New Age-like instrumentation, McKennitt’s arrangements are at once elegant and eloquent, enchanting and familiar, whether she’s singing “Coventry Carol,” “God Rest Ye Merry, Gentlemen,” or “Good King Wenceslas.” “Snow,” adapted from a 19th-century poem, and the …