(Exodus 35:3) commands: "You shall not light a fire in all of your dwellings" on the Sabbath. The Mekhilta connects this verse to a completely different discussion about the shemitah year, revealing an unexpected legal linkage.
(Exodus 34:21) says: "From plowing and harvesting shall you rest." The Mekhilta interprets this as: rest from plowing at the time of harvesting — meaning one must stop plowing on the eve of the shemitah year, even before the shemitah formally begins. You may not plow in the sixth year if the harvest would extend into the seventh.
Someone might extend this logic to the Sabbath. Just as the shemitah reaches back into the preceding year (you stop plowing before the shemitah begins), perhaps the Sabbath reaches back into Friday. If the Sabbath's rest extends backward, then any work done on Friday that continues into the Sabbath would be forbidden. You would have to cease all activity well before sunset.
"You shall not light a fire in all of your dwellings" prevents this extension. By specifying a particular labor (lighting fire) that is forbidden on the Sabbath itself, the Torah indicates that the prohibition applies to the Sabbath day, not to the preceding day. Unlike the shemitah, which extends backward, the Sabbath's labor prohibition begins precisely at its onset. Friday remains a work day until the Sabbath actually begins.