Jesus repeatedly taught that some day He would return -- and when He does, He will judge all the evils of this world, and establish His perfect rule of justice and peace. The Bible says God "has set a day when he will judge the world with justice by the man he has appointed" (Acts 17:31).
When will this happen? The Bible says God has not revealed the exact time to anyone -- not even the angels or Jesus Christ. Jesus said, "No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father" (Mark 13:32). (Incidentally, "the Son" clearly refers here to Jesus, who alone is God's Son; see, for example, John 5:22.)
As you say, in spite of this warning, occasionally some people have claimed to know exactly when Jesus was returning (although their predictions turned out to be wrong). Unfortunately, their failure often caused people to scoff at the Bible's teaching about Christ's return and deny its warning that some day this world will be judged. But we deny these truths at our peril -- for the day will come when every one of us must stand before God.
Although we don't know exactly when Christ will come again, the Bible does give us a number of signs pointing to His return. For example, it says He will return only after the Gospel has been preached throughout the earth -- and because of new means of communication, that day may be approaching (see Matthew 24:14). Are you ready for that day? You can be, by committing your life to Jesus Christ without delay.