
der Vortag
previous day
The day immediately before a specific, named day.
Am Vortag seiner Prüfung war er sehr nervös und konnte kaum schlafen.
On the previous day of his exam, he was very nervous and could hardly sleep.
previous day
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previous day
The day immediately before a specific, named day.
Am Vortag seiner Prüfung war er sehr nervös und konnte kaum schlafen.
On the previous day of his exam, he was very nervous and could hardly sleep.
The word is made up of the preposition 'vor' (before) and the noun 'Tag' (day). So it literally refers to the day 'before' a specific event.
Die Vorbereitungen für die Feier liefen schon am Vortag auf Hochtouren.
Preparations for the celebration were already in full swing on the previous day.
In most cases, 'Vortag' (previous day) is used in the fixed phrase 'am Vortag' (on the previous day) to indicate a point in time.
Am Vortag der Abreise hat sie noch schnell die Koffer gepackt.
On the day before departure, she quickly packed the suitcases.
'Vortag' (previous day) is a relative word. It always means the day before a specific date or event mentioned in the context, not simply 'yesterday'.
Die Konferenz begann am Mittwoch. Am Vortag gab es bereits ein informelles Treffen.
The conference began on Wednesday. On the previous day, there had already been an informal meeting.
yesterday
Gestern always refers to the day before today. Der Vortag, on the other hand, refers to the day before another day known from the context. Use gestern when looking back from today; when there is a stated reference day, use der Vortag.

Sie ist am Vortag der Sitzung angekommen.
She arrived the day before the meeting.
Sie ist gestern für die heutige Sitzung angekommen.
She arrived yesterday for today’s meeting.