For instance, the two-word phrase, "some day," consists of an adjective ("some") and a noun ("day") and refers to a single SPECIFIC day in the future. Although it's a specific day, you refer to it as "some day" when you don't know which specific day or you've forgotten it. Nevertheless, it specifically exists. (That's what the "some" in the phrase is doing--defining which day.)
"Someday," on the other hand, is a single-word adverb that refers to future events that will occur on a single day that is still indefinite or unknown in time. It's a nonspecific day because there is no adjective that can be inserted without breaking up "someday" into two words. "Someday" is a single non-modifiable adverb!
Confused? Here are two examples of "someday" in a sentence:
"Someday [an unspecific day], I will invest in a new laptop, but until then I will make do with the old one."
"She told me that I would be an adult someday [an unspecific day]."
The adverb in both these examples refers to an indefinite or unknown (unspecified) day in the future.
Now, here are two examples of "some day" in a sentence:
"I have a doctor’s appointment some day [a specific day] next month."
"She scheduled the meeting for some day [a specific day] in August, but she doesn’t remember when it is."
The adjective/noun phrase in these examples refers to a specific day in the future that has either been temporarily forgotten or perhaps never known (but nonetheless still exists!)
Simple? Yes and no. Think of the phrase, "some day," as if you inserted the word "SPECIFIC" between "some" and "day." You can't insert the word "SPECIFIC" into the single word "someday" without breaking the word in two and changing the usage!
Remembering that "some day" can be broken up with a specific modifier is a good tip for recalling which form to use when.
And if you forget, here's how your publisher will handle the problem: He'll hire an editor whose job it is to remember--at least he will someday.