Hebdomad

God was now to deliver them, from the bodily captivity of Babylon: ſo was he alſo after ſeventy hebdomades more, to deliver them from bondage of ſin and prevarication, and that by the anointed Meſſias, which is indeed the Holy of Holies. This (I ſay) may be the reaſon of naming ſeventy Hebdomades, thereby to allude to the number of the ſeventy years of that Babylonicall ſervitude. For that immediately after the Angel appointeth the whole exact number to be threeſcore and nine Hebdomades; that is, ſeven to the building of the City and Temple, and threeſcore and two, from that, to the death of Chriſt, in theſe words. [...]

Source: wiktionary

Let us now pass to the other species of the hebdomad, which is comprehended in the decad, and which exhibits an admirable nature no less than the former hebdomad. This, therefore, is composed of one, two, and four, which possess two most harmonic ratios, the duple and the quadruple; the former of which forms the symphony diapason, and the latter the symphony desdiapason.

Source: wiktionary

From the seventh of Artaxerxes to the death of Christ, are exactly seventy hebdomads complete. From the same epoch to the commencement of John's ministry, are exactly sixty-nine hebdomads. [...] From the coming of Christ, that is, from the first preaching of the Gospel, to the destruction of Jerusalem, are seven hebdomads; and this event happened exactly at the end of the first half of the seventh hebdomad, which "sacrifice and oblation" visibly ceased.

Source: wiktionary

The word "hebdomad" 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 hebdomad of days, (i.e. seven days,) or a hebdomad of months, (i.e. seven months,) or a hebdomad of years, (i.e. seven years.)

Source: wiktionary