# the word of Yahweh by ... Jeremiah

"what Yahweh spoke through Jeremiah"  

# until the land should ... its Sabbath rests

"until the land had rested according to the Sabbath law". The people were supposed to obey the Sabbath law by not farming the land every seventh year.   

# It observed its Sabbath for all ... desolation

"The requirements of the Sabbath law were fulfilled as long as the land lay desolate" 

# so that ... seventy years in this way

"so that 70 years might pass while the land lay desolate"