I'm agnostic myself, was raised Jewish though.
I looked it up and found this:
What does it mean that Jesus will return like a thief in the night? Is Jesus coming back like a thief in the night referring to the Rapture or the Second Coming?
www.gotquestions.org
The way I understand this passage, is the Bible compares his return to a thief in the night, which will catch unbelievers by surprise. If God were to tell people the exact day, then people would just get ready for that day and that day only, similar to how if a theif told people the day he was gonna rob them, they would get ready for that day only. Therefore you are supposed to be ready, as in practicing the Christian faith, at all times, so it doesn't catch you by surprise.