"Því seinna þennan sama daga" - What grammatical case is 'daga'?
Example sentence: "Því seinna þennan sama daga reyndi mamma að drepa sig með því að stökkva fram af bílastæðahúsi á Hverfisgötunni".

What grammatical case are these words in "þennan sama daga"?

I would expect this sentence to say "þennan sama dag" in the accusative singular without the final -a. "Daga" is either accusative or genitive plural.

Thank you in advance.

Question

"Því seinna þenn­an sama daga" - What grammatical case is 'daga'?
Example sentence: "Því seinna þenn­an sama daga reyndi mamma að drepa sig með því að stökkva fram af bíla­stæðahúsi á Hverf­is­göt­unni".

What grammatical case are these words in "þenn­an sama daga"?

I would expect this sentence to say "þenn­an sama dag" in the accusative singular without the final -a. "Daga" is either accusative or genitive plural.

Thank you in advance.

Answer

I think it's supposed to be "Því seinna þenn­an sama dag...."

daga only exists as a grammatical case of the plural form (as you thought)

In this context it would be accusative (þolfall)
:)