Or HaChaim on Exodus 19:12/אור החיים על שמות י״ט:י״ב

1 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.