a substantially rigid , segmented , flat sheet blank parallelogram , which , may be shaped and folded into a reversible three dimensional rectangular parallelepiped . the sheet includes six foldably connected side wall panels , each in the form of a parallelogram , preferably a square or rectangle . the sheet also includes additional material such as support panels , and / or border material which defines , with the six side wall panels , the sheet blank in the shape of a parallelogram of the present invention . in preferred embodiments the flat sheet blank parallelogram is rectangular in shape . in some embodiments there is no excess border material , while in still other preferred embodiments there are no support panels . in some embodiments the first display surfaces of the six side wall panels and the second display surfaces of the six side wall panels both carry indicia , preferably the six side wall panel first display surfaces each carry one of six consecutive calendar months and the six wall panel second display surfaces each carry one of the remaining six consecutive calendar months of a calendar year on its respective second display surface . the unfolded sheet may be sized to be mailable without an envelope . the sheet may be made of paper , poster sheet material , fiber sheet , card sheet , cardboard , and foldable plastic .