
I thought it would be a good idea to start talking about kanji. And what better way to start than just writing about the most common of them all?
That would be 日.
This is a 4-stroke kanji and you write it like this:

日 has several
kun readings: -か、 ひ and -び. The -か is a counter for days of the month e.g. 七日, pronounced なのか, means 'the seventh day of the month'... and also 'seven days'. -び is a typical ending for げつようび (monday), たんじょうび (birthday) etc.
日 also has two
on readings: one is にち, which we know from 毎日 (まいにち) or 'every day'. The other is じつ and I don't remember seeing it anywhere.
Important: The expression こんにちは, even though it contains the sound にち, is written using hiragana and doesn't use this kanji.
The meaning of 日 is 'sun' or 'day', and it usually means one of the two in combination with other kanji, except of course in the most common use, 日本 (にほん), Japan.
On its own, it's pronounced ひ. In combination with the days of the week (see above) it changes to び. The best one in the list is Sunday, 日曜日, which contains the same kanji twice, pronounced differently each time
にちよう
び.
And in the words for tomorrow, yesterday etc, it's impossible to tell which part of the sound it is:
今日 = きょう = today
明日 = あした = tomorrow, also pronounced あす.
昨日 = きのう = yesterday
明後日 = あさって = day after tomorrow
一昨日 = おととい = day before yesterday
This shows, more than anything, that you shouldn't think of kanji as sounds.
And here are some more combos:
日程 = にってい = schedule, program
生日 = たんじょうび = birthday
毎日 = まいにち = every day
日記 = にっき = diary