si est adverbium, multo minus iungis praepositionem. nam legimus praepositionem adverbio non iungi. numquid possum dicere ‘de mane’ et similia?
If it is an adverb, much less do you join a preposition [to it], for we read that a preposition is not joined to an adverb. Surely I cannot sayde mane and suchlike?[1]
AIS:Sprach- und Sachatlas Italiens und der Südschweiz [Linguistic and Ethnographic Atlas of Italy and Southern Switzerland] –map 347: “domani” – onnavigais-web.pd.istc.cnr.it