German phrase
An welchem ~ möchten Sie reservieren?
Meaning
What ___ would you like to make a reservation for?
When to use it
It asks what you want to book. This polite wording can clarify the day, time, course, or room.
Example dialogue
An welchem Tag möchten Sie reservieren?
What day would you like to make a reservation for?
Am Samstag, bitte.
Saturday, please.