Sentence:  In a thinly veiled threat to Saddam Hussein, President George Bush marked the six-month <WSD>anniversary</WSD> of September 11th by reiterating America's commitment to prevent rogue countries obtaining weapons of mass destruction. 
Meaning of the word marked with <WSD> tags:  (loosely) A day subsequent in time to a given event by some significant period other than a year (especially as prefixed by the amount of time in question).