Numbers 28:8
And the other lamb shalt thou offer at even: as the meat offering of the morning, and as the drink offering thereof, thou shalt offer it, a sacrifice made by fire, of a sweet savour unto the LORD. And the other lamb shalt thou offer at even: as the meat offering of the morning, and as the drink offering thereof, thou shalt offer it, a sacrifice made by fire, of a sweet savour unto the LORD.
The law appointed a regular evening lamb as part of the continual sacrifices, so that an offering was kept burning on the altar. That evening service was an expression of continual gratitude, showing humanity's ongoing obligation to God under the old dispensation.
The lamb slain in the evening symbolized the death of the Saviour as the Lamb of God who must die for the sins of humanity. Jesus understood in His child-life that this evening lamb pointed to Himself and His own appointed death.