Sun, Day
noun
The kanji and the word are exactly the same. That means they share meanings as well. Take note that this word means both "sun" and "day," though. Why? because the sun takes a day to go around and come back!
ひ
Since this word is made up of a single kanji, it uses the kun'yomi reading. Here's a mnemonic to help you to remember it: What do you feel from the sun? You feel the heat (ひ).
そろそろ日が出ますよ。
The sun will come out soon.
子犬がくる日は三日です。
The date the puppy comes is the 3rd.
日本にきた日にパスポートをなくした。
I lost my passport on the day I came to Japan.