Why wait 56 days before donating blood again?

Because your body takes time to replace the red blood cells. When donating whole blood, the plasma (clear fluid), white blood cells and platelets that have been 'harvested' are replaced within a couple of days. The red blood cells are manufactured within the bone marrow of the donor and take much longer for the body to replace. It also gives your body time to recover from the donation. Donating blood at shorter intervals would leave you anaemic.