The Mosaic covenant is the covenant that God made with Israel through Moses:

This covenant compiles up the giving of the Torah (the written law) and Oral law which was handed over to Israel through Moses, during the season of Shavuot (Pentecost) which took place in the third month called Sivan on the Hebrew.

Exodus 19:1 in the 3rd month called Sivan on God’s calendar:

[ Israel at Mount Sinai ] In the third month after the children of Israel had gone out of the land of Egypt, on the same day, they came to the Wilderness of Sinai.

Exodus 19:3-8 (NKJV)
3 And Moses went up to God, and the Lord called to him from the mountain, saying,

“Thus you shall say to the house of Jacob, and tell the children of Israel:

4 ‘You have seen what I did to the Egyptians, and how I bore you on eagles’ wings and brought you to Myself.

God commands Israel to live by obedience:

5 Now therefore, if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me above all people; for all the earth is Mine.

God makes a proposal of making Israel into a kingdom of priests: 

6 And you shall be to Me a kingdom of priests and a holy nation.’ These are the words which you shall speak to the children of Israel.”

7 So Moses came and called for the elders of the people, and laid before them all these words which the Lord commanded him.

The children of Israel accepts God’s proposal:

8 Then all the people answered together and said, All that the Lord has spoken we will do….

18 Now Mount Sinai was completely in smoke, because the Lord descended upon it in fire.

Its smoke ascended like the smoke of a furnace, and the whole mountain quaked greatly.

The Shofar sound is blown by God on the mountain on Pentecost (Shavout):

19 And when the blast of the trumpet sounded long and became louder and louder, Moses spoke, and God answered him by voice.

The picture of Rapture – God comes down on the mountain and then Moses goes up:

20 Then the Lord came down upon Mount Sinai, on the top of the mountain. And the Lord called Moses to the top of the mountain, and Moses went up.