I think it was the 14th but I do not think they were eating the lamb that night especially when compared to John 18:28. I think this was 13/14 and preparatory toward the lambs being slaughtered the next afternoon.
Based on what? Blblically it was the 14th.
Jesus body was placed in that tomb after sun down, Geneva Bible, "And nowe when the night was come (because it was the day of the preparation that is before the Sabbath) . . . ." Mark 15:42. The Jewish days begin with the evening, night fall.
BTW, the Passover, Luke 22:1 then referred to the week, Numbers 28:24, ". . . this manner ye shall offer daily, throughout the seven days, the meat of the sacrifice made by fire, of a sweet savour unto the LORD: it shall be offered beside the continual burnt. . . ."
Deuteronomy 18:1, ". . . The priests the Levites,
and all the tribe of Levi, shall have no part nor inheritance with Israel: they shall eat the offerings of the LORD made by fire, and his inheritance. . . ."
John 18:22 ". . . Caiaphas unto the hall of judgment: and it was early; and they themselves went not into the judgment hall, lest they should be defiled; but that they might eat the passover. . . ."