etymonline.org wrote:
Old English
dæg
"period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic
*dages-
"day" (source also of Old Saxon, Middle Dutch, Dutch
dag
, Old Frisian
di
,
dei
, Old High German
tag
, German
Tag
, Old Norse
dagr
, Gothic
dags
), according to Watkins, from PIE root
"a day." He adds that the Germanic initial
d-
is "of obscure origin." But Boutkan says it is from PIE root
*dhegh-
"to burn" (see
). Not considered to be related to Latin
dies
(which is from PIE root
*dyeu-
"to shine").
Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English
Wodnesniht
was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.
From late 12c. as "a time period as distinguished from other time periods."
"daily" is from late 14c.;
"all the time" is from late 14c.
"day away from work" is attested from 1883;
first recorded 1897. The
days
in
nowadays
, etc. is a relic of the Old English and Middle English use of the adverbial genitive.
"something unusual taken as routine" is by 1820. The nostalgic
is attested by 1907.
, expressing mild doubt following some boast or claim, is by 1941. To
"stop working" is by 1919; earlier
call it a half-day
(1838).
"at some day in the near future" is from late 15c.
"a day of misfortune" is by 1936.