Some Day vs. Someday
Both “someday” and “some day” are established. Someday is a one-word adverb referring to an unknown future time, while the open form also exists and can be the ordinary determiner-plus-noun construction “some day.”
“Someday” is an established adverb; “some day” is also valid, especially when “some” directly modifies “day.”
Key details
Choose the right form
| Meaning / grammatical role | Form | Decision |
|---|---|---|
| one-word adverb meaning at an unknown future time | someday | Use the established adverb. |
| ordinary determiner + noun construction where “some” directly modifies “day” | some day | Use the open phrase. |
| an unspecified future day where both spellings may be encountered | follow the intended grammar and house style | Do not invent a universal semantic split when both forms can point to an unspecified future time. |
Quick tests
Meaning / construction test
Is the expression functioning as the established one-word adverb meaning “at an unknown future time”?
If yes: Use “someday.”
If no: Use “some day” when “some” directly determines the noun “day.”
Caveat: Both forms are established; context and grammatical structure matter more than a made-up absolute meaning difference.
Examples
Someday I hope to learn another language.
“Someday” functions as the one-word adverb for an unknown future time.
Some day next week should work for the delivery.
“Some” directly determines the noun “day.”
Choose some day that suits everyone.
The open form is the ordinary determiner-plus-noun construction.
Further guidance
Core difference
“Someday” is an established one-word adverb for an unknown future time; “some day” is also an established two-word expression and can additionally function as determiner plus noun.
Usage Boundary
Do not invent a universal meaning difference when both forms refer to an unspecified future day; follow the governing style and use the open form when “some” directly determines the noun “day”.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- someday — Cambridge Dictionary (opens in a new tab)Cambridge Dictionary · Meaning and spelling of someday as an adverb for an unknown future time
- one/some day — Cambridge Dictionary (opens in a new tab)Cambridge Dictionary · Established two-word some day construction for an unspecified future time