The second aliyah: Since the ten commandments were given in Parshat Yisro, G-d has been dictating the Torah to Moshe up on mount Sinai and made for him a miraculous set of luchos (tablets). When Moshe went up to the mountain to receive all this, he told the people to expect him back in forty days. But the people erred in their calculation by a few hours and, with the help of the evil inclination and the bad influence of the mixed multitude that were with them, they grew impatient and made a golden calf.
Moshe's brother Aharon the high priest, cooperated in the making of the idol only to the extent that he thought that they would be delayed long enough for Moshe to return.
The women didn't participate in contributing to, or the making of the idol, and as a reward, G-d made Rosh Chodesh, the beginning of every month, a holiday for women.
Moshe finally comes down, sees what has transpired, and breaks the luchos. Rashi says that the luchos were like a marriage contract, and the idol worship was like adultery, so breaking the "marriage contract" made the "adultery" not quite as bad as if the marriage contract were still in force. Then Moshe appeals to G-d to be merciful to the Jews. G-d does forgive them, and assures them that they will be given the land flowing with milk and honey as originally promised their forefathers. However, G-d says that they will be lead by an angel and not by Him, as was done previously.