昨日 and 今日: anchoring yesterday and today
Identify dates relative to the speaking day and distinguish completed events from plans.
Events yesterday
昨日 (きのう) means the day before the speaking day. 「昨日、本を借りました」 says the speaker borrowed a book yesterday.
In 「昨日借りた本を返します」, borrowing happened yesterday, while returning is a separate event. Do not attach 昨日 equally to every action.
Past and future within today
今日 (きょう) means today. 「今日、友達に会いました」 says the speaker has already met a friend today.
「今日、友達に会います」 can state a plan to meet a friend today. 今日 itself does not establish completion; read the verb and context too.
Keeping the reference date
「昨日は雨でしたが、今日は晴れています」 contrasts yesterday's rain with today's clear weather. Do not merge the two dates.
今日 in an older message refers to the day when it was written. It does not automatically become the reader's current day several days later.
Which action happened yesterday in 「昨日借りた本を今日返します」?
Show the answer and why
Borrowing the book. Returning it is separately scheduled for today.