When calculating the number of Sabbath days in the 6,000 years and adding the fact that the 1,000-year interval before the advent of עולם הבא is composed only of Sabbath days we get a total of 600,000 Sabbath days, or the same number as the number of the Jewish souls which experienced the Revelation and the building of the Tabernacle. This number also equals the number of letters the Torah is made up of.
-Shenei Luchot HaBerit Torah Ohr 6