Of the Question Whether There is Any Contradiction Between Matthew and Mark on the One Hand, and John on the Other, in So Far as the Former State that After Two Days Was to Be the Feast of the Passover, and Afterwards Tells Us that He Was in Bethany, While the Latter Gives a Parallel Narrative of What Took Place at Bethany, But Mentions that It Was Six Days Before the Passover
152 Matthew continues thus: “And it came to pass, when Jesus had finished all these sayings, He said unto His disciples, You know that after two days will be the feast of the passover, and the Son of man shall be betrayed to be crucified.” This is attested in like manner by the other two—namely, Mark and Luke,— and that, too, with a thorough harmony on the subject of the order of narration. They do not, however, introduce the sentence as one spoken by the Lord Himself.
They make no statement to that effect. At the same time, Mark, speaking in his own person, does tell us that “after two days was the feast of the passover and of unleavened bread.” And Luke likewise gives this as his own affirmation: “Now the feast of unleavened bread drew near, which is called the passover;” that is to say, it “drew near” in this sense, that it was to take place after two days' space, as the other two are more apparently at one in expressing it. John, on the other hand, has mentioned in three several places the nearness of this same feast-day.
In the two earlier instances the intimation is made when he is engaged in recording certain matters of another tenor. But on the third occasion his narrative appears clearly to deal with those very times, in connection with which the other three evangelists also notice the subject—that is to say, the times when the Lord's passion was actually imminent.
Source: The Harmony of the Gospels (New Advent)