In this week's Torah portion, Vayechi,

In this week's Torah portion, Vayechi, we read that before Jacob passed away he called all his children together. He told them, "Gather yourselves together, that I may tell you that which will befall you in the last days." But we find that Jacob did not reveal the future. Our Sages explain that the Divine Presence left Jacob, and with it, the knowledge of the end of days. In other words, G-d prevented Jacob from revealing the date of the Final Redemption.We can be sure that Jacob did not intend merely to satisfy their curiosity; rather, Jacob thought that he would be doing the Jewish people a favor by revealing when the end of days would come. However, this revelation would not have brought any tangible benefit, but would have actually caused Jacob's descendants harm: Had the fledgling Jewish nation learned that the Final Redemption was not to come for thousands of years, they would have despaired. Why would Jacob want to reveal something that would have caused them to despair?
In order to explain what really took place, we must understand the two different ways in which the Redemption can come about. The Talmud teaches that although G-d has fixed a definite date in history for Moshiach to come, if the Jewish people is worthy, Moshiach will arrive before the appointed time. If the Jews, through their actions, merit the Redemption sooner, they will not have to wait until the specified date.
Jacob did not plan on revealing the final date by which time the Redemption would have to occur; he wanted to give his children a far closer one by which time Moshiach would come if they so merited. It is quite possible that Jacob was referring to a time only a few years after that very day. Why was this not allowed to happen?
The answer lies in the fact that had Jacob been successful in revealing the date, the Redemption would have had to occur then. Once the Divine Presence was withdrawn from him, the opportunity to speed the process was taken away, and we still await the Redemption today.
Had the Jewish people known that the Redemption was so near, they would surely have perfected their actions and been worthy. The knowledge would have encouraged and inspired them to remove all obstacles to Moshiach's arrival, and we would not still be waiting.
Obviously, this was not meant to be. G-d prevented Jacob from revealing the secret, for the Final Redemption must come about purely on the merit of our efforts, working within the limitations placed on us by the physical world. Jacob's revelation would have affected the quality of man's worship and changed the Divine plan. G-d therefore caused His Presence to depart from Jacob.
Jacob, for his part, knew all this, but tried to hasten the Redemption anyway. Although G-d has ordained otherwise, the request of a tzadik (righteous person) is never in vain and the ramifications of Jacob's actions are felt today: Every Jew must demand that G-d bring the Final Redemption in our own time, and this very insistence will infuse us with determination in our mission. When we say, "We want Moshiach now" and "Moshiach is on the way," we automatically improve our behavior in our desire for the Redemption to finally come.
Adapted from the works of the Lubavitcher Rebbe