Sorry to come back to the issue of the relationship between the Synoptics again, but it does affect whether Matthew's source was an eyewitness of Jesus or someone from a later generation.
One particular evidence sometimes cited for Mark's dependence on Matthew and Luke is his use of dualisms: in particular, sentences or phrases in which Matthew has one half and Luke has the other half. A well known example is Mark 1:32, "And evening having come, when the sun had set" (cf Mt 8:16, "And evening having come"; Lk 4:40, "Now when the sun was setting"). If Luke and Matthew were copying from Mark, it would be less likely that on many occasions one of them copied one half while the other of them copied the other half of Mark's sentence than that Mark was conflating the phrases he found in Matthew and Luke.
While reading The Making of Mark I've come across many examples like these, and in Mark 14 we have a few:
Mk 14:1, "Now it would be the Passover and unleavened bread after two days"
Mt 26:1, "You know that after two days the Passover takes place"
Lk 22:1, "Now the feast of unleavened bread, called Passover, was drawing near".
Mk 14:1, "and the chief priests and scribes were seeking how, by stealth..."
Lk 22:2, "and the chief priests and scribes were seeking how..."
Mt 26:4, "...by stealth...
Mk 14:12, "And on the first day of unleavened, when they were to sacrifice the passover lamb"
Mt 26:17, "Now on the first of the unleavened"
Lk 22:7, "Then came the day of the unleavened, on which it was necessary for the passover lamb to be sacrificed"
This happens throughout Mark's gospel, so frequently that it points strongly to Mark conflating Matthew and Luke.
Matthew, then, was the first to write about the Last Supper. And John gives us the correct chronology with regard to its date.
Whether or not Matthew meant that the last supper happened on the evening of the official Passover, his account is the earliest.