The fourteenth of the first month — the night the firstborn were spared, kept every year since.
Passover is kept on the fourteenth of Nisan, and it is the one feast whose instructions were given before there was anything to remember: Israel was told how to keep it on the night it happened. A lamb without blemish, its blood on the doorposts, the meal eaten in haste and ready to travel. Every later Passover is a re-enactment of that night, which is what makes the timing of the crucifixion — during the feast, with a lamb's bones unbroken — the connection the gospels keep pointing at.