"Exodus 24:16 And the glory of the LORD abode upon mount Sinai, and the cloud covered it six days: and the seventh day he called unto Moses out of the midst of the cloud" (DLT: torahtimes.org).

and the seventh day: Seven days after Shavuot, when the ten commandments are spoken, the LORD calls to Moses.  This is the second Sabbath in the month of Sivan, Sivan 13.  The counting is from immediately after the 70 elders went up the mountain, which was on the Sabbath of Shavuot (Pentecost). (DLC: torahtimes.org)

Daniel's Literal Translation and Commentary: (http://www.torahtimes.org/translation/exo2416.html)