Sentence:  The number 666 has long held significance . . . that it's a mark of the devil. But now that the actual <WSD>calendar day</WSD> is about to arrive — on Tuesday, 6/6/06 — it's not only the superstitious among us who are looking ahead nervously. 
Meaning of the word marked with <WSD> tags:  A day having a conventional designation on a recognized calendar, such as a numerical identification within a named month.