late 14c., "daily, happening every day," from Late Latin diurnalis "daily," from Latin dies "day" + -urnus, an adjectival suffix denoting time (compare hibernus "wintery"). Dies "day" is from PIE root *dyeu- "to shine" (source also of Sanskrit diva "by day," Welsh diw, Breton deiz "day;" Armenian tiw; Lithuanian diena; Old Church Slavonic dini, Polish dzień, Russian den).
From early 15c. as "performed in or occupying one day;" 1620s as "of or belonging to the daytime (as distinguished from nocturnal). Related: Diurnally.
The meaning "book for inventories and daily accounts" is from late 15c. in English (14c. in French); that of "personal diary" is c. 1600, also from a sense developed in French. Meaning "daily publication" is from 1728. Classical Latin used diurnus for "of the day, by day," and also as a noun, "account-book, day-book."
Initial -d- in Latin usually remains in French, but according to Brachet, when it is followed by an -iu-, the -i- becomes consonantized as a -j- "and eventually ejects the d." He also cites jusque from de-usque.