A day is defined as a period of darkness followed by an equal period of Light, whether on Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune of Pluto..
Jesus died at 3 PM on Thursday after a 3 hour period of Darkness from 12 noon to 3 PM.
The three hour period of light, before being buried at dusk, being day one: Friday night--night one, Friday day---day two, SATURDAY NIGHT----Night two----- Saturday, day three, Sunday night----night three.
Three days and three nights. But the question was not, "On which day of the week did Jesus die," but did he die on the day of Passover or the first day of the seven day festival of Unleavened Bread.