Which is correct?
The message from human resources asked if she could come for an interview __________ next week.
b. some day
“Someday” refers to an unspecified time in the future, as in “Someday my prince will come.” As two words, “some day” refers to an unspecified but specific day: The interview will be scheduled for a particular but unspecified day the following week. “Someday” is incorrect in that context.
Confusingly, you can substitute “some day” for situations where “someday” works — but not vice versa.
Grammarist explains more about “someday” vs. “some day.”