When Mose went up into Biblical mount Sinai to receive the Ten commandments (Exodus 24:12-18), he left the Israelites for forty days and forty nights. The Israelites feared that Moses would not return, and they decided to demanded that Aaron the priest make them “gods” those liken unto to the ones they Egyptians worshiped so tht they may go before them (Exodus 32:).

Aaron gathered up the Israelites’ golden earrings and ornaments, constructed a “molten calf” and they declared: “These[be] thy gods, O Israel, which brought thee up out of the land of Egypt.” (Exodus 32:4)

According to the Bible, the golden calf (עֵגֶּל הַזָהָב ‘ēggel hazāhāv) was an icon (a cult image) made by the Israelites during Moses’ absence, when he went up to Mount Sinai. In Hebrew, the incident is known as ḥēṭ’ ha‘ēggel (חֵטְא הַעֵגֶּל) or “The Sin of the Calf”. It is first mentioned in Exodus 32:4.

 Bull worship was common in many cultures. In Egypt, whence according to the Exodus narrative the Hebrews had recently come, the Apis Bull was a comparable object of worship, which some believe the Hebrews were reviving in the wilderness.

Moses went down from the mountain, but upon seeing the calf, he became angry and threw down the two Tablet of Stone, breaking them. Moses burnt the golden calf in a fire, ground it to powder, scattered it on water, and forced the Israelites to drink it. When Moses asked him, Aaron admitted collecting the gold, and throwing it into the fire, and said it came out as a calf  ( Exodus 32:21-24).

Moses returns on Mount Sinai to seek God’s mercy for 40 days:

During the month of Elul Moses was directed by God’s Spirit to return back on Mount Sinai, to recieve a new set of the Tablets containing the Ten commandments, and for 40 days Moses repented, prayed and sought God’s mercies for Israel.

On The 40th day (which fell on the 10th of the month of Tishrei) when Moses returned, he found the children of Israel separated and sanctified, keeping themselves holy as they wanted for Moses to return.