What are the chances behind what items you get in chests/boxes in Counter Strike: GO?

Obviously people say it comes down to luck and everything, but there must be percentages behind what item you get. Otherwise, it will be completely random and valve may end up have people get more knife's than the more common items (Which in that case, make knives more common items).

So does anyone know or have some sort of a clue on the percentages on the rarities of an item?

Also, are you more likely to get a rare item at a less peak time or does that not have any effect on what you get? (What I mean by this is say if at 6pm everyone was opening cases, are you less likely to get a rare item at this time or does this have no effect on what you get?)

Are items decided on what they are going to be before you open the case? (So say I was about to open a case. Is the item that is going to come out going to be decided before I open it or chosen through the percentile method once opened?)

My Dream Knife. (Hoping to get it as Factory New, lol.):

The chances are determined once you open/unbox. The cases don't have pre-existing loot that you unbox.

And no, the time of the unbox does not affect your chances.

The chances have not been officially confirmed by Valve.