Sentence:  The word "<WSD>"hebdomad</WSD>"" stands in the same relation to "seven", that ""decad"" does to "ten", or our English word ""dozen"" to "twelve". As therefore we can say a dozen of days, or a dozen of months, or a dozen of years, so we can say, a <WSD>hebdomad</WSD> of days, ("i.e." seven days,) or a <WSD>hebdomad</WSD> of months, ("i.e." seven months,) or a <WSD>hebdomad</WSD> of years, ("i.e." seven years.) 
Meaning of the word marked with <WSD> tags:  A group of seven.