Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
some time, someday, some day, sometime, sometimes
Some time is an adjective-plus-noun combination: We still have some time left before dinner. Sometime is both an adverb meaning “approximately,” as in He’ll arrive next week sometime [or sometime next week], and an adjective meaning either “former” or “occasional,” as in My mother was a sometime office-holder in our town and She had a passion for the theater and a sometime interest in the cinema. In the phrase a sometime thing, however, the meaning seems to have drifted beyond “occasional” to a mildly pejorative sense meaning “on-again-off-again,” with the implication that sometime things are fairly unstable.
Some day is the normal adjective-plus-noun construction, meaning “one day, but not a particular day.” Someday is an adverb, working much as the adverb sometime does: Someday [Sometime] soon, we must have lunch together.
