The Collaborative International Dictionary of English v.0.48:

Yom \Yom\, n. [Heb. y[=o]m.]
   Day; -- a Hebrew word used in the names of various Jewish
   feast days; as, Yom Kippur, the Day of Atonement; Yom Teruah
   (lit., day of shouting), the Feast of Trumpets.
   [Webster 1913 Suppl.]