There are probably plenty of reasons why we are not allowed to know the day or the hour of the return of Yeshua for his Church. I know at least two very important reasons:
1. We are supposed to be on the look-out every single day; if we would know it's e.g. May 21, 2011 or April 7, 2012, we might become complacent during the time prior to the event, whereas the Lord wants us to stay in touch with Him and be a watchman each and every day;
2. If we would know the date, then the enemy would know too (he's much smarter then we can even imagine) and he could use that information to his advantage. We're involved in spiritual battle; it's spiritual warfare here and you do not want the enemy to know such a strategic date on God's calendar.