This may be either,
1. A precept, as it is generally understood, that none should ever come at those times without some offering or other, for the support of the Levites, and of the worship of God; but the determination of this, or what they would give, was left to their choice. Or,
2. A promise to encourage them to come so oft from their remotest habitations to Jerusalem, because they should never appear before God in vain, i.e. to no purpose, or without some benefit, for so the word rekam oft signifies. So it may be parallel to Isaiah 45:19, I said not unto the seed of Jacob, Seek ye me in vain. But the former sense is more probable, by comparing this with its parallel place, Deuteronomy 16:16,17.