A.T. Robertson Commentary Mark 14:12

"And on the first day of unleavened bread, when they sacrificed the passover, his disciples say unto him, Where wilt thou that we go and make ready that thou mayest eat the passover?" — Mark 14:12 (ASV)

When they sacrificed the passover (οτε το πασχα εθυον). Imperfect indicative, customary practice. The paschal lamb (note πασχα) was slain at 6 P.M., beginning of the fifteenth of the month (Exodus 12:6), but the preparations were made beforehand on the fourteenth (Thursday). See on Mt 26:17 for discussion of "eat the passover."