Sentence:  The interesting thing about these ceremonies is that they must have originated in a period when the Romans were using true lunar months based on the observation of the crescent moon. The <WSD>Kalends</WSD> then would have been the day after the evening on which the crescent had been first sighted, the nones 
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.