Jamieson, Fausset and Brown on Exodus 12:6

6. keep it up until the fourteenth day, &c.--Being selected from the rest of the flock, it was to be separated four days before sacrifice; and for the same length of time was Christ under examination and His spotless innocence declared before the world.

kill it in the evening--that is, the interval between the sun's beginning to decline, and sunset, corresponding to our three o'clock in the afternoon.