It usually doesn't matter if you buy a given stock immediately or two weeks later. Sure, the stock could move up or down but typically not enough to matter all that much.However, there are exceptions. Sometimes, a stock has a potential catalyst right around the corner. In such a case, investors could be much better off buying sooner rather than later.Vertex Pharmaceuticals (VRTX 3.06%) might be one of those special cases. Should you buy Vertex stock hand over fist before Jan. 30?A key date for VertexWhat's ...