also: さくじつ
Refers to the day before today. It can be used as a noun, as in 昨日の夜, or adverbially before a clause. きのう is the ordinary spoken reading; さくじつ is more formal.
It rained all day yesterday.
I met a friend yesterday.
A new policy was decided at yesterday's meeting.
I talked with a friend on the phone last night.
I've had a headache since yesterday.
It had been raining until yesterday.
Practical kana spelling for the everyday reading, often used when a softer or less formal style is desired.