The 14th does. They came to Him about it. It does not say that occurs before. It says the 14th came and they asked Him. They could have asked Him on the 13th because that night was the beginning of the 14th (the day when the lambs were killed). Read Mark 14:12 in the original Greek. The word "pro" is used which means before.
protos doesn't mean before when it's modifying a noun. It's a superlative, and it means very first, first-most, etc. It's like hard, harder, hardest. By context, it would be like saying it was the firstest day. It modifies "day" as being one in a series. There's no such thing as a before day. But there is a first day in a series of days. But barring the grammatical argument, it doesn't account for the actions of the disciples, who "made ready the passover," which they couldn't have done on the before day.
Nevertheless, I find it ironic how you can take some of the most ambiguous statements and declare them as factually stating something they don't state, but when something states something unambiguously, you have difficulty grasping it.
They didn't ask him on the 13th. They asked him on the "first day of unleavened bread (which isn't the 13th), when the passover must be killed (which isn't the 13th)," where he wanted them to prepare for him to eat the passover.
Let me restate. The gospels state that THEN CAME THE DAY OF UNLEAVENED BREAD, WHEN THEY KILLED THE PASSOVER. They don't say, Then came the day before they killed the Passover. They don't say, Then came the day when they started searching for leaven to remove the next day. On the first day of unleavened bread, when the passover had to be killed, they came to him and asked where he wanted them to prepare for him TO EAT THE PASSOVER.
So, stop trying to conflate the scriptures. As I already pointed out, they say what they say. The first day when unleavened bread had to be eaten was the 14th, from noon forward. The day when the passover had to be killed was the 14th, at 3:00 in the afternoon. ON
THAT DAY, which is the 14th, the disciples came to him asking where they wanted him to prepare for him to
eat the Passover. It was LATER,
in the evening of THAT DAY, that they sat down and ate the last supper.
To restate the original challenge, please demonstrate credibly how this is
not the 14th in the Synoptic accounts.