Kommentar zu Schemot 19:12

והגבלת AND THOU SHALT SET BOUNDS — set boundary marks for them as a sign that they should not approach the mountain from the boundary and beyond.

כל הנוגע בהר מות ימות, a warning to deter the people from being harmed in the event they would trample the boundary markers around the mountain in their eagerness to catch a glimpse of G’d. Such people, if they were to become victims of their own folly, would mar the entire joyful experience of the giving of the Torah by defiling the earth around them with their carcasses. They would cause their relatives to become mourners on what should have been the most auspicious experience of their lives. The presence of G’d, the Shechinah, does not manifest itself to people in mourning.

והגבלת את העם סביב, "set a boundary for the people all around." It appears that the instructions to set up this fence applied only for what would take place on the third day; it was meant so that the Israelites would get used to the fact that the Mountain was out of bounds and as a measure of respect for what would happen. It does not mean that touching the Mountain or its edge was forbidden already on the two days preceding the revelation. Mechilta 24 confirms that the obligatory nature of this prohibition applied only on the third day.