Exodus 12:6

And ye shall keep it until the fourteenth day of the same month, separated from the rest of the flock, in order to keep it from contamination and infection; and the whole assembly of the congregation of Israel, all the housefathers as household priests and representatives of the children of Israel, shall kill it in the evening, literally, "between the two evenings," at twilight. Custom later fixed the time at between three in the afternoon and sundown.