If trading is halted for your pick at the market open on the first day of the contest or at the market close on the last day of the contest your pick is disqualified. The stock must be buyable at the market open and sellable at the market close at the end of the week, on Friday.
Your pick must be unique. First poster to make a pick gets it. You may choose the same stock as another player, however not the same position.
A brief rationale for your pick makes the contest more fun.