The difference between “someday” and “some day”
Writing for Business
Which is correct?
The message from human resources asked if she could come for an interview __________ next week.
a. someday
b. some day
Answer: b.
Explanation:
“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.”