How should I store salmon en croute?
You will obviously need to refrigerate.  If it is only 12-hours, I would cool, then leave it uncovered in your refrigerator, or maybe a loose draping of plastic.  For longer storage, I would wrap well and freeze.  Reheat in a 425-450F (218 - 232C) oven until just warm in the center (test with a cake tester by inserting into thickest part, wait 5- 10 sec., and touch to your lip). The pastry should come back, but probably not to its original glory.