Numbers 6:12
And he shall consecrate unto the LORD the days of his separation, and shall bring a lamb of the first year for a trespass offering: but the days that were before shall be lost, because his separation was defiled. And he shall consecrate unto the LORD the days of his separation, and shall bring a lamb of the first year for a trespass offering: but the days that were before shall be lost, because his separation was defiled.
This requirement is part of the instructions for completing a Nazarite vow. When the separation is defiled, the days already fulfilled are lost and do not count toward the vow. The defiled person must consecrate the days of separation anew, beginning the whole period of consecration over again. That renewal requires bringing a lamb of the first year for a trespass offering.
The vow also involved purification ceremonies. Those ceremonies included costly sacrifices. This law applied to four men whose Nazarite vows were nearly complete and who still had purification sacrifices to offer.