Sentence:  The third day before the <WSD>calends</WSD> of February is 30 January, the third <WSD>calends</WSD> of March is 27 or 28 February, and the third of the <WSD>calends</WSD> of May is 29 April. 
Meaning of the word marked with <WSD> tags:  "Often with initial capital": the first day of a month, particularly  the first day of a month of the Roman calendar.