"Unde ai dormit luni?"

Translation: Where did you sleep on Monday?

[JonEnMiljon]

”Where did you sleep on Monday” sounds like incorrect English to me. I'm not a native English speaker, but I would say ”last Monday” in this case.

[impvam]

..On Monday is perfectly acceptable english usage.
Last Monday is OK too, but the subtlety would be given in the sentence's context or word emphasis.
If you were to ask me "Where did you sleep last Monday", I would assume you were seeking to compare it with some other Monday, or some other particular day.

Leaving out the 'on' as PaulMinden suggests, sound unnatural to a British English speaker - it's a more American English construct (although you will often hear this form in Scotland too).

[PaulMinden]

You're right that ON Monday sounds unnatural. I would probably just say, "where did you sleep Monday?" Or, more likely, "where did you sleep Monday night?" to specify LAST Monday is probably overkill, since the past tense tells us it's in the past, and not next Monday.

[JonEnMiljon]

Thanks for clarifying! Prepositions are cool, but it's even cooler when you don't have to use them :)

[starvinp]

You can use ON when you ask. Where do you sleep on Mondays?

[JonEnMiljon]

Yes, but not in the past tense, like the sentence here.

[Lng52-._]

I see nothing wrong with "on Monday"; e.g., "Where did you sleep on Monday?" (this past Monday).