And on the first day of Unleavened Bread, when they killed the Passover lamb...
Mark 14:12

Human language is rarely precise. Never make the mistake of reading Scripture as if it's a computer program in which every word has only one possible meaning. In Mark 14:12 "the first day of Unleavened Bread" doesn't mean first day of the week of Unleavened Bread, but the first day of the official ceremonies, which is actually the day *before* Unleavened Bread, when the Passover lamb is killed.

This isn't an error in the text. It's just a reflection of how real people speak of things. For those who celebrate Christmas, there's only one Christmas Day and yet there are also the "Twelve Days of Christmas". People and Scripture also use the term "Passover" to refer to the day before Unleavened Bread begins, or the full week of Unleavened Bread, or the weeks leading up to Passover, or just only the Passover lamb, as in this verse.