Difference between 日 and 一日


What is the difference between 日{ひ｝ and 一日｛いちにち}? I thought that 一日 was used
when refering to the **length** of a day but I saw this sentence and got
confused:

> 今朝のニュースでも、アナウンサーの人が「うららかな **一日** になるでしょう」って言ってたよ。
>
> Also, on the news this morning the announcer said that it would probably
> become a beautiful day.

Why is it correct to use 一日 rather than 日 here? Thanks.




The concept of a day is quite ambiguous in any language and Japanese is no
exception. A day can be: 24 hours, a calendar day, a period between sunrise
and sunset, a period between you wake up and go to sleep, one's workday. One
day might have more than 24 hours (長い一日だった), the other less...

That said 一日{いちにち} in your example refers to a time period that someone
perceived or might perceive as a "beautiful day". It is essentially 一日間 here
and although using it would be unnatural, it would probably be grammatical.

Like in 忙しい一日でした the 間 is absent, but if you extended the period to two weeks
you would say 忙しい2週間でした.

日{ひ} on the other hand has more of a "calendar day" or an "exemplary day"
feeling to it. うららかな日は外で遊びます。




一日 means either the first day of the month or one day, but can also mean all
day.

In that context, 一日 is used to denote that the **whole** day will be
beautiful.

Anyone please correct me if I'm wrong, I'm still learning myself.

