Did the world end on May 21?

1 Thessalonians
So, apparently the world was supposed to end on May 21.  That didn't happen...  My question is, why do people think they can accurately predict when the world will end?  It says right here in 1 Thessalonians 5:2 "for you know very well that the day of the Lord will come like a thief in the night."  We don't expect thieves to come in the night, and when they do, they're a surprise (unless you live in a very crime-ridden city).  That doesn't mean we can't be prepared for the eventuality.  When they do come, we won't be caught off guard.  But we won't know for certain when they'll come.
It's the same way with Christ's return.  We know he's coming back, that much is certain.  But when he'll come, we don't know, as was clearly evidenced by his non-return on May 21.  Not knowing when he'll come should not be worrisome, however.  If we're living in faith, knowing that our salvation comes through Christ alone, it won't matter when he comes back because regardless, we'll be ready.  And that's a relief!