vorgestern
the day before yesterday
On the day before yesterday.
Wir sind vorgestern aus dem Urlaub zurückgekommen.
We came back from vacation the day before yesterday.

the day before yesterday
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the day before yesterday
On the day before yesterday.
Wir sind vorgestern aus dem Urlaub zurückgekommen.
We came back from vacation the day before yesterday.

The word is simple in structure: it consists of 'vor' (before) and 'gestern' (yesterday). Literally, it therefore means 'the day before yesterday'.
Ich habe dich vorgestern in der Stadt gesehen.
I saw you in town the day before yesterday.
As an adverb, 'vorgestern' (the day before yesterday) can stand at the beginning of the sentence (position 1), followed by the verb in position 2. But it can also stand after the verb and the subject.
Vorgestern hat es den ganzen Tag geregnet.
The day before yesterday, it rained all day.
'Vorgestern' (the day before yesterday) is often combined with times of day to specify the time more precisely. Typical combinations are 'vorgestern Abend' (the evening before last), 'vorgestern Morgen' (the morning before last), or 'vorgestern Nachmittag' (the afternoon before last).
Wir haben uns vorgestern Abend zum Essen getroffen.
We met for dinner the evening before last.
today
'heute' and 'vorgestern' are both time adverbs for specific days. But they mean different days. 'heute' is the day on which you are speaking. 'vorgestern' is two days before today, that is, the day before yesterday. They can be confused if you quickly jump between days when telling a story. Pay attention to the direction in time: 'heute' is now, while 'vorgestern' lies in the past.

Mila hat vorgestern auf der Feier viele alte Freunde gesehen.
Mila saw many old friends at the party the day before yesterday.
Heute fährt die Fahrerin wegen des Wetters besonders vorsichtig.
Today the driver is driving especially carefully because of the weather.