# The days are coming when

"A time is coming when" or "Soon" 

# you will desire to see

"you will want to see" or "you will wish to experience"

# one of the days of the Son of Man

"one of the days when the Son of Man will rule as king"

# the Son of Man

Jesus is speaking about himself. Alternate translation: "I, the Son of Man"

# but you will not see it

"you will not experience it"