gestern
yesterday
On the day before today.
Ich habe dich gestern in der Stadt gesehen.
I saw you in town yesterday.

yesterday
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yesterday
On the day before today.
Ich habe dich gestern in der Stadt gesehen.
I saw you in town yesterday.

As a time adverb, 'gestern' (yesterday) can stand flexibly in the sentence. It often stands in position 1, followed by the verb. But it can also stand in the middle of the sentence after the subject and verb.
Gestern war ich den ganzen Tag zu Hause.
Yesterday I was at home all day.
Since 'gestern' (yesterday) refers to the past, the verb in the sentence must always be in a past tense, either the Perfekt or the Präteritum.
Wir sind gestern Abend ins Kino gegangen.
We went to the movies yesterday evening.
To give a more precise time on the previous day, 'gestern' (yesterday) is often combined with times of day. Typical expressions are 'gestern Morgen' (yesterday morning), 'gestern Mittag' (yesterday at noon), 'gestern Abend' (yesterday evening), or 'gestern Nacht' (last night).
Wir haben uns gestern Abend zum Essen getroffen.
We met for dinner yesterday evening.
The idiom 'von gestern sein' (to be outdated) is a fixed construction. It is used to describe a person, an idea, or an object as old-fashioned.
Seine Ansichten über Mode sind wirklich von gestern.
His views on fashion are really outdated.
today
'heute' and 'gestern' both refer to days directly around the moment of speaking. 'heute' is the current day. 'gestern' is the day before. The words are opposites with respect to the near time frame and can easily be confused, especially when talking about appointments or experiences. Ask yourself: If it happens on the current day, it is 'heute'. If it already happened on the previous day, it is 'gestern'.

Gestern war es in der Nachbarschaft wegen des Sturms sehr laut.
Yesterday it was very loud in the neighborhood because of the storm.
Ich habe der Kundin gesagt, dass ich sie heute nach der Arbeit anrufe.
I told the customer that I would call her today after work.