I found this sentence:
What is the meaning of that か in 「事だかわからない」？ Is it there to add a "uncertainty" nuance? or is it something else?
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This か is a question marker, and いつの事だか is an embedded question.
As the above link says, it's okay to have だ before か in embedded questions, and the が between the noun and わかる is usually dropped.
I saw a dream that night. I don't remember when I saw it, but there was a scenery I do remember.