Someday means at some time in the future, not yet fixed. Someday soon, someday I will, someday we'll know.
It carries hope and vagueness together, which is why it sounds like a promise nobody has to keep. Written as one word it is the adverb; some day as two words means a particular day not yet named. The distinction is thin and often ignored.
1adverbsomedayAt some unfixed time in the future.
“Someday I'd like to sail around the world in my own boat.”
someday is a time.
some + day.
Etymology from Wiktionary, CC BY-SA