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.
17 And the sight of the glory of the Lord was like devouring fire on the top of the mount in the eyes of the children of Israel.
18 And Moses went into the midst of the cloud, and gat him up into the mount: and Moses was in the mount forty days and forty nights.
The presence of the Lord descended upon the mountain, but Moses did not go up into it straightaway. He waited for the Lord to call him up, which did not occur until the seventh day. This immediately calls to mind the six days of creation, and the seventh day of rest. The reason for this parallel is not explained.
Perhaps that seventh day coincided with the Jewish sabbath, and the Lord was waiting for that sacred day to call Moses apart from the world. Perhaps the cloud on the mountain was purifying the place before the Lord’s arrival, recreating that part of the earth over six days just like it had taken six days to perform the original creation. Perhaps Moses required the six days to properly prepare his own soul for the meeting. Whatever the reason, we see a pattern of waiting a full measure for the time to be right.
This idea of sacred things taking a full measure to complete is also present in Moses then being up in the mountain for forty days and forty nights. If waiting seven days to ascend calls to mind the Genesis story of creation, then staying up in the mountain for forty days calls to mind Noah shut up in the ark while it rained for forty days and forty nights.
Both the initial conception of the earth and the flood are creation stories. Initial creation and recreation after the first had gone astray. They are symbols of beginning and resetting, of making everything new. Perhaps that was the Lord’s intent with these numbers, to suggest that His communion with Moses would usher forth a new beginning for Israel and all the world, a recreation of laws and principles that had been lost.