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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.