(Not a teacher)
By fronting the time element(On Sunday), it is usually for emphasis. If not, it should be placed in the proper order as this:
When I got up at 7 o'clock on Sunday, I found that no one was at home.
When the subordinate clause is put before the main clause, a comma is normally required.
When I got up at 7 o'clock on Sunday(subordinate clause), I found that no one was at home(main clause).
Otherwise, no comma is needed:
I found that no one was at home(main clause) when I got up at 7 o'clock on Sunday(subordinate clause).

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