The word first can also mean "before" or "the beginning." The word translated as "day," heeméra, can refer to a literal 24-hour period of time, but it can also indicate a general period of time or a season (see Luke 9:51; 17:24; 19:42; 23:7; John 8:56; Acts 2:20; 8:1; 17:31; Romans 2:5; I Corinthians 3:13; II Corinthians 6:2; Ephesians 6:13; Hebrews 3:8). So the first part of Mark 14:12 could also be translated, "Now at the beginning of the season of Unleavened Bread . . ." or "Now at the beginning of the time of Unleavened Bread . . ." Nothing dictates that in this case heeméra designates a specific 24-hour period, and much argues against it.
As the notes for Matthew 26:17 show, the disciples asked this question before the Feast of Unleavened Bread and before they had kept the Passover. But how are we to understand the explanation, "when they killed the Passover lamb"? In the Greek, the word translated as "killed" is éthuon. It can indeed refer to the singular act of slaying an animal (Acts 11:7), but also to a religious sacrifice (Acts 14:13) or to the entire occasion of which a slaughtered animal was paramount, such as the fatted calf being killed for the prodigal son (Luke 15:23, 27, 30). In addition, in the sentence in question, the verb tense indicates an action in progress but not yet completed.
In other words, the sacrificing of the Passover lamb—or preparations for doing so—was taking place at the time the disciples asked their question! Remember, most of the people did not observe a Temple-kept Passover; in Mark 14:12, the common people were sacrificing lambs throughout the city, not the priests. The priests would not slay the Temple Passover lambs until the following afternoon. But as Abib 14 was drawing near, the disciples observed people around them on the outskirts of Jerusalem in the process of sacrificing—at least engaging in the necessary preparations, even if they did not perform the sacrifice itself until after sunset—prompting them to ask Jesus where He wanted them to likewise prepare for Passover.
David C. Grabbe