dma_va,
Some INS offices stamp passports only on specific days, like NYC office will do it only on Fridays, Newark on Wednesdays etc. Others keep it open since they are not as busy as the above mentioned ones. The approval notice (original and cc) specifies any restrictions commanded by the local INS office. So the appointment day (not date). If your approval notice does not specify one, then you can go any day and get it stamped.