We must admit at once that this doctrine has, in the past, led Christians into very great follies. Apparently many people find it difficult to believe in this great event without trying to guess its date, or even without accepting as a certainty the date that any quack or hysteric offers them. To write a history of all these exploded predictions would need a book, and a sad, sordid, tragi-comical book it would be. One such prediction was circulating when St. Paul wrote his second letter to the Thessalonians. Someone had told them that “the Day” was “at hand.” This was apparently having the result which such predictions usually have: people were idling and playing the busybody. One of the most famous predictions was that of poor William Miller in 1843. Miller (whom I take to have been an honest fanatic) dated the Second Coming to the year, the day, and the very minute. A timely comet fostered the delusion. Thousands waited for the Lord at midnight on March 21st, and went home to a late breakfast on the 22nd followed by the jeers of a drunkard.
Clearly, no one wishes to say anything that will reawaken such mass hysteria. We must never speak to simple, excitable people about “the Day” without emphasizing again and again the utter impossibility of prediction. We must try to show them that that impossibility is an essential part of the doctrine. If you do not believe our Lord’s words, why do you believe in his return at all? And if you do believe them must you not put away from you, utterly and forever, any hope of dating that return? His teaching on the subject quite clearly consisted of three propositions. (1) That he will certainly return. (2) That we cannot possibly find out when. (3) And that therefore we must always he ready for him.
C.S. Lewis, "The World's Last Night" (1960)