日
Kun readings 🇯🇵
On readings 🇨🇳

The kanji derives from the pictogram that originally represented the solar disk and which in its evolution has taken its current form. Each word that contains 日 has to do with the day of the year on the calendar, and also with sunlight.
This ideogram has 4 画 traits and is also radical, meaning that it becomes a part within other kanji, like a keyword of the kanji itself.
明るい is an adjective which means bright
, clear
. As you can see the kanji 明 contains the element 日 "Sun, light"
日本 Nihon or Nippon is the term that indicates Japan: the Rising Sun, 本 in this case indicates the origin
The adjective "Japanese" derives directly from the Japanese name and pronunciation.
日曜日 Sunday, the day of the Sun
All terms indicating the day of the week contain the ideogram of day 日 at the end of the word:
月曜日
Monday
火曜日
Tuesday
水曜日
Wednesday
木曜日
Thursday
金曜日
Friday
土曜日
Saturday
日曜日
Sunday
曜日 indicates the day of the week.
誕生日 the birthday, the day (日) of the birth (誕生)
For the birthday there is also often the loan word from English "birthday", which is adapted to Japanese phonetics and becomes バースデイ.
お誕生日おめでとうぎざいます。
Happy birthday

時 is the ideogram that translates time, moment, hour, season and contains as radicals the element 日 day, say
日 is also counter, classifier or numeric suffix to count the days of the month:
一日
the first of the month
二日
the two
三日
three
四日
four
五日
five
六日
you are
七日
seven
八日
eight
九日
nine
十日
ten
十一日
eleven
十二日
twelve
十三日
thirteen
十四日
fourteen
十五日
fifteen
十六日
sixteen
十七日
seventeen
十八日
eighteen
十九日
nineteen
二十日
twenty
二十一日
twenty-one
二十二日
twenty-two
二十三日
twenty-three
二十四日
twenty-four
二十五日
twenty-five
二十六日
twenty-six
二十七日
twenty-seven
二十八日
twenty-eight
二十九日
twenty-nine
三十日
thirty
三十一日
thirty-one
Here above we see the kanji of the numbers. It's important to consider that Arabic numbers are widely used also in the Japanese language.
日 is also the kanji of the famous city of Nikkō: 日光市, "the city of sunlight”, in the mountainous region of Tochiji prefecture

生年月日 we need to fill in some documents where the date of birth is requested: 生 birth, 年 year, 月 month, 日 day.
日付 is the term instead that simply refers to the "date".
本日 indicates today, today's date. 翌日 the next day, the next day.
毎日
Every day
Other adverbs of time that contain 日:
今日
today
昨日
yesterday
一昨日
the day before yesterday
明日
tomorrow
明後日
after tomorrow
元日 is the first day of the year, January 1st; 大晦日 instead indicates the last day of the year, December 31st
日 as a radical also in kanji 智 wisdom, reason and intelligence

A beautiful holiday in Japan is Nature Day which falls on the Golden Week period, the holiday week between late April and early May. 緑の日 the day of greenery which is celebrated in May.
お日様 is a beautiful term for the Sun
; the kanji 日 of sun / day is preceded by a prefix お and followed by a suffix 様, both honorific to emphasize the greatness and gratitude to the Star to whom we owe our Life
暇 free time, rest, relaxation contains the radical 日

良い一日を!
Have a nice day!
Example of kunyomi readings 🇯🇵
ひ
The reading ひ (hi
) is the standard reading for the word day
and sun
and all the expressions that contain these words: every day
, one day
, number of days
, sunlight
.
It is also used in words like 春日 spring
, 朝日 morining sun
, and 夕日 evening sun
.
外に出るともう夕日が沈んでいた。
If you go outside the evening sun was already setting down.
び
The reading び (bi
) is used for the days of the week among other things:
日曜日:
Sunday
土曜日:
Saturday
平日:
weekday
Example of onyomi readings 🇨🇳
にち
The reading にち (nichi
) is used for time frequencies (everyday
, all day long
) and for some relative dates (today
, tomorrow
, first day
).
じつ
The reading じつ (jitsu
) is similar to にち. It is used for words about a time duration or to specify a relative date: today
, tomorrow
, yesterday
, previous day
, half day
, and such.
今日は本当にありがとうございました。
Thank you so much for today.