newspaper holiday; day without newspaper publication
Refers to a scheduled day when a newspaper is not published, such as a weekly holiday or a special non-publication day.
明日は休刊日なので、新聞は届きません。
Tomorrow is a newspaper holiday, so the paper won't be delivered.
この新聞の休刊日は日曜日です。
This newspaper's day off is Sunday.
Compound of 休 (rest, suspend) + 刊 (publish) + 日 (day). The exact historical derivation is uncertain, but the word transparently means 'day of suspended publication'.