Moses was requested to ask the Egyptian King, Pharaoh, to let the people of Israel go out from Egypt and worship their lord in the desert.
After a long negotiation that concludes the ten plagues, the sons of Israel leave Egypt in the middle of the night, after 210 years of hard slavery in an impressive event known as the Exodus to celebrate the ecstatic freedom in an event called Pessach פסח, or Passover.
Moses leads the sons of Israel through the desert and the red sea, and after 49 days from the Exodus the sons of Israel are standing against Sinai Mountain ready to receive the open code of human behavior, the Torah.
Moses goes up the mountain to bring the boards of covenant and the sons of Israel are worrying about him, for Moses stays on the mountain 40 days and 40 nights: being afraid from his absence, the people are asking Aaron, his brother: “What has happened to the man who took us out of Egypt?” Aaron as an act of protection builds for them a golden calf, an object that can be seen and observed by the senses, something that the people can point at and say: “Those are your Gods, Israel, that brought you up from Egypt”(Exodus 34, 4 ) (אֵלֶּה אֱלֹהֶיךָ יִשְׂרָאֵל, אֲשֶׁר הֶעֱלוּךָ מֵאֶרֶץ מִצְרָיִם (שמות לד ד:
From the mountain Moses noticed the pagan celebration and God wished to destroy them for not believing in human spirit and going back to slavery of material.
Moses with the help of the tribe of Levi stops the leadership of those who follow the worship of the golden calf and rises up to the mountain to ask for forgiveness: Moses is asking for forgiveness 40 days and 40 nights from the beginning of the Month of Elul until the 10th of the month of Tishrei, when finally he comes down from the mountain with the second boards of covenant and the divine forgiveness.
To remind ourselves of the divine forgiveness we celebrate every year on the tenth of Tishrei Yom kippur which is considered to be a day of self examination and self criticism, we see when we were wrong, where we can improve, and basically that we forgive ourselves and others, because love cover all sins (Proverbs 10:12)
וְעַ֥ל כָּל־פְּ֝שָׁעִ֗ים תְּכַסֶּ֥ה אַהֲבָֽה, משלי י יב