1Yesterday — the day before today. Read 'きのう' in everyday speech, and 'さくじつ' in more formal or written contexts.
昨日は仕事の後、友達と食事に行った。
Yesterday, after work, I went out to eat with a friend.
昨日買った靴を今日から履いている。
I've been wearing the shoes I bought yesterday since today.
昨日の夜、急に雨が降ってきた。
It suddenly started raining last night.
昨日から頭痛が続いている。
I've had a headache since yesterday.