someday, some day

Written as one word, someday is an adverb meaning at some indefinite time:

            Someday my prince will come.

Written as two words, the adverbial phrase (adjective and noun) means a somewhat more definite time and is the correct form to use after a preposition:

            Lesley will schedule the next meeting for some day in the middle of September.

See also: sometime, sometimes, some time.