(Exodus 35:2) says: "And on the seventh day it shall be holy for you." The Mekhilta explains why this clarification was needed. Israel might have reasoned as follows: the daily offering is brought on the Sabbath, and this involves labor — slaughtering, burning, and handling the sacrifice. If labor for the Temple service is permitted on the Sabbath, perhaps labor outside the Temple is also permitted.
The Torah blocks this logic: "It shall be holy for you." The Sabbath's holiness applies to you — to your personal domain, your home, your field. In your sphere of activity, labor is forbidden. But for God — meaning for the Temple service — the Sabbath is "mundane" in the sense that the normal Sabbath restrictions on labor do not apply.
This creates a sharp distinction between sacred and ordinary labor on the Sabbath. The Temple has its own rules. The sacrificial service continues because it operates in God's domain, where different principles apply. But the individual Israelite, in his own domain, observes the Sabbath with full force. What is permitted inside the Temple walls is forbidden outside them.
The Mekhilta prevents a dangerous slippery slope. If Temple labor were allowed and no boundary were drawn, people might gradually extend the Temple's exception to their own activities. The Torah draws the line: the Temple is God's sphere, where His service overrides the Sabbath. Everything outside the Temple is yours, and there the Sabbath is fully holy.