1. The beginning of the month is a day when work is permitted. However, the women have a custom not to work on this day (apart from household work such as baking and cooking which are definitely permitted) and it is a good and correct custom. However, those men who follow custom have erred as for them it is not a custom at all, and they are permitted to work. (Shulchan Aruch and Mishne Berurah 417)
2. The reason for this distinction between men and women is that since at the time of the “sin of the golden calf” in the desert the women did not allow their jewellery to be taken by their husbands for making the golden calf and Hashem had given them as a reward in return for this that they keep the beginning of the months more than men. (ibid)
