Es kommt mir vor, als sei heute Sonntag.
Es kommt mir vor, als wäre heute Sonntag.
Most of the time, I see wäre used in an als construction like this, but I come across sei once in a while. How can I determine when to use one or the other?
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On the one hand, the grammatical functions of the two subjunctive moods are briefly as follows:
Going by this, only wäre (subjunctive II) is correct as the subordinate clause is describing an irreal situation; actually it’s not Sunday. There is no productive grammatical function of the subjunctive I that applies here and which would call for sei (subjunctive I).
Here is some prescriptivist sharing this point of view.
On the other hand, using the subjunctive I in such constructions (irreal comparisons) is so common that one could consider it correct by usage. After all, languages evolve and are shaped by their speakers. If enough people make a mistake, it is not a mistake anymore. I am not aware of any difference in meaning between the two subjunctive moods in this case.
Here is a descriptivist observation by the Duden supporting this point of view.