What is First Day of Issue (首日封)?
The first day of issue is the day on which a postage stamp, postal card or stamped envelope is put on sale. Sale is only on the day itself. Once the day is over, the stamps can no longer be purchased.
A first day cover is an envelope whereupon postage stamps have been canceled on their first day of issue. There will be a first day of issue postmark, usually a pictorial cancellation, indicating the city and date the item was first issued.
In this example, my Tiger Zodiac stamps are canceled on 8th Jan 10 with a Taiga mark. Once its is canceled, the stamps cannot be used anymore. This pre-cancellation is highly sought after by stamp collectors.
This one here is the presentation pack. Comes with the same 3 stamps. Makes a great gift to friends and comrades. Give one to your boss and he might just give you a raise. ^^;