Albert Barnes Commentary Leviticus 6:5

"or any thing about which he hath sworn falsely; he shall even restore it in full, and shall add the fifth part more thereto: unto him to whom it appertaineth shall he give it, in the day of his being found guilty." — Leviticus 6:5 (ASV)

In the day of his trespass offering — The restitution was thus to be associated with the religious act by which the offender testified his penitence.