Meaning of “day” - Learner’s Dictionary

day

noun uk us /deɪ/
24 HOURS [ C ]

A1 a period of 24 hours:

the days of the week
January has 31 days.
Matthew runs five miles every day.
I saw her the day before yesterday.
LIGHT HOURS [ C, U ]

A2 the period during the day when there is light from the sun:

a bright, sunny day
We've been travelling all day.
These animals sleep during the day and hunt at night.
WORK HOURS [ C ]

A2 the time that you usually spend at work or school:

She's had a very busy day at the office.
the other day

B1 a few days ago:

I saw Terry the other day.
day after day

every day for a long period of time:

Day after day they marched through the mountains.
one day

A2 used to talk about something that happened in the past:

One day, I came home to find my windows smashed.
one day/some day/one of these days

B1 used to talk about something you think will happen in the future:

One of these days I'll tell her what really happened.
days

used to talk about a particular period of time when something happened or existed:

in my younger days
This book was written before the days of computers.

B1 a long time:

I haven't seen Jack for days.
these days

A2 used to talk about the present time:

I don't go out much these days.
in those days

B2 used to talk about a period in the past:

In those days, no-one had a TV set.
the old days

a period in the past

→ See also April Fool's Day , Boxing Day , Christmas Day , at the end of the day , field day , Independence Day , Mother's Day , New Year's Day , open day , polling day , Valentine's Day , call it a day , it's early days , make sb's day , save the day

(Definition of “day” from the Cambridge Learner’s Dictionary © Cambridge University Press)