"Ab" is considered Dative preposition, but in Duden it is said it sometimes can come with Akkusative: "Bei artikellosen Substantiven mit adj. Attribut gelegtl. auch mit Akk.", what does exactly this mean?
Does it mean when noun is without article, after "ab" case can be Akkusative?
According to the "Zweifelsfallduden" (Duden Band 9, Das Wörterbuch der sprachlichen Zweifelsfälle), the preposition ab in conjunction with locations always requires the dative:
ab unserem Werk
ab welcher Station?
ab allen deutschen Flughäfen
In all other use cases, ab can also be used with the accusative:
ab erstem Mai / ab ersten Mai
ab letztem Montag / ab letzten Montag
ab vier Nächten / ab vier Nächte
Ab 50 Exemplaren / Ab 50 Exemplare wird Rabatt gewährt.
Dieser Film ist jugendfrei ab zwölf Jahren / ab zwölf Jahre.
However, if the statement is expressed with a determiner, again only the dative is possible:
ab dem 15. Mai
ab der dritten Runde
ab meinem 18. Lebensjahr
The abbreviation gelegtl. is for gelegentlich – sometimes.
Sie ist ab nächster Woche in Urlaub. (Dativ)
Sie ist ab der nächsten Woche in Urlaub. (Dativ)
Sie ist ab nächste Woche in Urlaub. (Akkusativ)
I account this more to general sloppyness than being a real Akkusativ. On word ends, -er is spoken Tiefschwa and -e is spoken Schwa, there isn't that much difference.
I mistrust the analysis of Duden here.
It's true that you can say "ab nächsten Donnerstag", which is definitely an accusative. But I don't think it's the "ab" which triggers the accusative here. Rather, "ab" can be used with any temporal expression: "ab morgen", "ab 10:00 Uhr", "ab Montag". And absolute temporal expressions use the accusative even without a preposition: "Nächsten Montag bin ich im Urlaub".
So the case is not due to "ab", but due to the use of a noun phrase as a temporal expression. (Note that "bis" and "von...an" exhibit the exact same behaviour with time expressions.)
