Some people are very interested in these "first day covers."

有些人对“首日封”非常感兴趣。

youdao

We not only have the souvenir folders, but also have the first day covers.

我们这儿不但有纪念邮折，还有首日封。

youdao

Used or unused postage or revenue stamps, stamp-postmarks, first-day covers, postal stationery (stamped paper), and the like, other than those of heading No. 49.07.

使用过或未使用过的邮票、印花税票、邮戳印记、首日封、邮政信笺(印有邮票的纸品)及类似品，但税目49.07的货品除外。

youdao

But it says more than 50, 000 Doctor Who packs, or first day covers, are likely to be sold.

BBC: The extreme stamp collecting of first day covers

So how do first day covers work, and who is buying them?

BBC: The extreme stamp collecting of first day covers

For them it is all about the first day covers.

BBC: The extreme stamp collecting of first day covers