It is possible in English (although not always strictly grammatically correct!) to place a preposition at the end of a question. In spoken English therefore you might hear "for which date is this?" or "which date is this for?". This second option does not exist in German as the preposition must be the first element in a question followed by the interrogative as second element. If the interrogative is not invariable (i.e. if its endings can change), then the case of the interrogative - and thus its endings - is determined by the preposition.
We have encountered the following combinations of questions and prepositions in this and previous chapters:
The German flag Grammar 13: Questions involving prepositions
Welcher?
Auf welchen Namen gehen die Zimmer?
("In which name should I book the rooms?")
Für welches Datum?
("For which date?")
Von welchem Gleis fährt der Zug?
("From which platform does the train leave?")
Mit welchem Bus fährst du?
("Which bus are you travelling on?")
Invariable interrogatives
Um wie viel Uhr wird das Frühstück serviert?
("At what time is breakfast served?")
Von wann bis wann?
("From when to when?")
