An event which is not nationally important enough to be commemorated by issue of a Commemorative/Special stamp or for which a stamp cannot be accommodated in the annual issue programme may be commemorated by the Department by issue of Special Covers which can be cancelled with a special cancellation at the post office selected for the purpose. Such covers can also be issued at the instance of private parties on payment of requisite charges.
These covers should bear minimum postage stamps of any kind equivalent to the minimum inland letter mail rate. Heads of Circles are the Competent Authority to sanction Special Covers commemorating any event/institution/inaugural flight under their jurisdiction. They also coordinate its release.
Source: India Post portal