As Boston winter-holiday traditions go, the Handel and Haydn Society’s “A Bach Christmas’’ doesn’t have the cachet of “Messiah’’ or “The Nutcracker’’ or Holiday Pops. But what would Christmas be without Bach’s Advent and Christmas cantatas, and in particular his “Christmas Oratorio’’? The program that American conductor Steven Fox, in his H&H debut, has assembled for this year’s celebration opens with Bach’s Cantata No. 133, “Ich freue mich in dir,’’ and closes with Cantata V from the “Christmas Oratorio.’’