what "paying back it's Sabbaths" means
According to Jewish law, every 7 years the land should not be cultivated. Israelites were required to let the land rest. The Israelites failed to this throughout the years. The scriptures don’t say exactly how many times the Israelites broke this law but 70 years of desolation was the total time that God required to allow the land to rest from being cultivated and harvested to pay back or compensate for the times this law was trespassed.
It is important to take note of this because some people can argue that God let the Jews out earlier than 70 years and reconcile 586 but that cannot apply to the land. Therefore, if 539 was the end of the 70 year desolation of the land, the desolation must have begun 70 years earlier.
“21 to fulfill Jehovah’s word spoken by Jeremiah, until the land had paid off its sabbaths. All the days it lay desolate it kept sabbath, to fulfill 70 years.” —2chr 2: 21
ALL the days