To fulfil the word of the Lord by the mouth of Jeremiah
That is, the Jews were so long servants in Babylon, as in the preceding verse, to accomplish Jeremiah's prophecy of it, ( 2 Chronicles 25:12 ) ( 27:7 ) ( 29:10 )
until the land had enjoyed her sabbaths;
the sabbatical years, or seventh year sabbaths, which, according to the law of the land, was to rest from being tilled, ( Leviticus 25:4 ) , which law had been neglected by the Jews, and now, whether they would or not, the land should have rest for want of persons to till it:
for as long as she lay desolate she kept sabbath, to fulfil threescore
and ten years;
as threatened in ( Leviticus 26:34 Leviticus 26:35 ) on which text Jarchi observes, that at the destruction of the first temple the law concerning the sabbath, or rest of the land had been neglected four hundred and thirty years, in which space were sixty nine sabbatical years; and, according to Maimonides F4, it was at the end of a sabbatic year that the city and temple were destroyed, and so just seventy years had been neglected, and the land was tilled in them as in other years, and now it had rest that exact number of years; but of this we cannot be certain, though it is probable.
The Brenton translation of the Septuagint is in the public domain.