“It is a mystery”
He gave the instruction a good year before the last supper. When was it instituted if it was to be taken literally and of eternal importance?
There is no mystery about the instruction. Jesus explains what he means that by “eating...and drinking” one has life.
53 Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.
63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
68 Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life.
It is by hearing his words, understanding and believing his words that one has life.
Indeed, but He was naturally present with them at the time. The Institution actually occurred at the Last Supper with the commandment given the holy Apostles, "do it". The discourse on the Eucharist in John 6 relates it to the Ascension (John 6:63), as it is at that time that the Eucharistic Presence of Christ became a necessity for us.