The account reads: "And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation. Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you. And he said unto the disciples, The days will come, when ye shall desire to see one of the days of the Son of man, and ye shall not see it. And they shall say to you, See here; or, see there: go not after them, nor follow them. For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in his day." (Luke 17: 20-24.)
When the kingdom of God reappears on the earth, men like the Pharisees will not be able to observe it. It will not come in a way observable to those outside. Instead, the kingdom will be "within" those who are included. For them, the "observation" comes from the Lord making Himself known to them. (See Numbers 12: 6.) But such experiences are not public. So for the Pharisees, it will be "without observation."
For His disciples, however, He went on to explain the time would come when they would want another day with Him, but that time will have passed. They may wish it, but the time to draw near Him is while He is available; not after His departure. When He has departed, then it will not be possible to find Him among those who say He is here! Or, He is there!
Though He may make Himself known on the same conditions to any who prepare themselves to receive Him, when He appears again it will be as lightening which lights heaven itself from one end to the other. It will be entirely unmistakable.
The Pharisees were interested in the subject because they envied the kingdom of heaven. They wanted to own it. They wanted to control it. If they could locate it, and usurp it, then they would have power over others who sought it.
Christ's answer completely frustrates man's capacity to control the kingdom of heaven. It is not a franchise, giving Pharisees any authority or right. It is not even capable of being "observed" by those who are blind to its appearance. But it is nevertheless real. It is "within" some few who qualify.