Throughout the book of Exodus, whenever the Israelites traveled, the Torah uses the plural form — "they journeyed," "they encamped" — because the people moved in discord and settled in discord. Quarreling was their default state. But when Israel arrived at the foot of Mount Sinai, something extraordinary happened. The Torah switches to the singular: "and it encamped" (Exodus 19:2), as though the entire nation were one person.
The Mekhilta reads this grammatical shift as a theological statement. For the first time since leaving Egypt, the Israelites were of one heart. Every other encampment had been fractured by complaint, rebellion, or internal division. Here, standing before the mountain where they would receive the Torah, they achieved a unity so complete that Scripture could describe them as a single entity.
The midrash (rabbinic interpretive commentary) also notes that God told the people they would remain at Sinai for an extended period. And indeed, the Israelites camped at the base of the mountain for nearly a full year — twelve months minus ten days. This was no brief stop on the journey. Sinai was a prolonged dwelling, a place where the nation was forged through shared revelation and shared law.
Finally, the phrase "opposite the mountain" is interpreted as meaning "to the east of the mountain." Wherever Scripture uses the word "opposite" (neged) in a spatial sense, the Mekhilta understands it as indicating the eastern side. The Israelites positioned themselves facing the rising sun, awaiting the dawn of a new covenant.