@opusdei & Smiddy
knowledge of the culture accounts for that discrepancy. Passover was celebrated as the festival of unleavened bread, and was a seven day observance. The first day and last days were the most special, but here is where it gets confusing for us looking back: technically, the 15th of Nisan (first day of Passover) began on the 14th after dusk. I can look that up for you all if you need, I have a reference I'd need to find in a book I have about the Jewish culture.
So that is why if you look in the accounts of Matthew and mark, it'll say it began on the first day of the festival of unleavened bread. This is not the same as Passover, but was the day the Passover sacrifice was "customarily" offered. Luke 22:21 explains this day was CALLED Passover, but it wasn't actually Passover. Not until dusk, at which point the next day begins.
also, technically John doesn't say they observed the meal the day before, it says Christ knew the day before "the festival" of the Passover - not the day before Passover (John 13:1). Whereas Matt 26:2 shows Jesus saying "two days from now IS PASSOVER". The difference here is this: the festival began on the 14th, the Passover sacrifice was offered, and families prepared to celebrate the Passover after dusk (which according to the Jewish calendar was the beginning of the next day. The 15th, which was Passover). So where matthew wrote Christ saying Passover was in two days would have been Nisan 13th. Where John wrote that he knew the day before the festival would have been a reference to the 13th, saying that he knew (compare matt 26:2), but john was not saying they observed Passover on a different day, only that Jesus knew before Passover that his "hour had come."
I know now that's confusing but it's true.