首日封是在新邮票发行首日，将该套邮票的全套或单枚贴在特制信封右上角，加盖当天邮政日戳或特别纪念邮戳的信封。经邮政部门实际寄递的称为首日实寄封。

首日封是邮票发行首日的记载，它与邮票密切关联，由邮政部门、集邮公司设计、印制和发行，也有集邮者自制的。

首日封一般都印有与邮票有关的图案和文本说明，以加强邮票的宣传作用。

现存世界上最早的首日封是英国1840年5月6日开始使用黑便士邮票时寄出的信封。

1957年11月7日，中国人民邮政发行《伟大的十月社会主义革命四十周年》纪念邮票，当时的中国邮票总公司（现称中国集邮总公司）首次印制、并正式发行了相关纪念邮票主题的首日封一套。自此，中国集邮公司开始根据中国邮政每年的邮票发行计划，有系列、有目的地发行各类（纪念、特种邮票）首日封和（记事）纪念封。

A first day of issue cover or first day cover (FDC) is a postage stamp on a cover, postal card or stamped envelope franked on the first day the issue is authorized for use within the country or territory of the stamp-issuing authority. Sometimes the issue is made from a temporary or permanent foreign or overseas office. There will usually be a first day of issue postmark, frequently a pictorial cancellation, indicating the city and date where the item was first issued, and "first day of issue" is often used to refer to this postmark. Depending on the policy of the nation issuing the stamp, official first day postmarks may sometimes be applied to covers weeks or months after the date indicated.