"They shall not appear before Me empty-handed" — the Torah requires that the pilgrims who come to the Temple on the three festivals must bring something. But what? The Mekhilta says: sacrifices. Not money, not gifts — animal sacrifices.
One might think "not empty-handed" means bringing money to donate to the Temple treasury. The Mekhilta rejects this through a verbal comparison. "Rejoicing" is associated with the pilgrim's appearance, and "rejoicing" is also associated with peace-offerings (shelamim) elsewhere in Scripture. Just as there — in the context of peace-offerings — the term refers to sacrifices, so here the "not empty-handed" requirement refers to sacrifices.
The pilgrim must bring animal offerings, not cash contributions. The festival appearance is not a fundraising event. It is a sacrificial encounter. The person must arrive at the Temple with something living — an animal to be offered on the altar and partially consumed in a communal feast.
This ruling ensures that the three annual pilgrimages maintained their character as religious encounters rather than economic transactions. The pilgrim's obligation was not to enrich the Temple but to participate in the sacrificial system — to bring an animal, watch it offered, and share in the meat with family and friends. The "non-emptiness" the Torah demanded was spiritual, expressed through the physical act of sacrifice.