IPA: /ˈsʌmˌdeɪ/
KK: /sʌmˈdeɪ/
At an unspecified time in the future.
I hope to travel the world someday.
Someday → The word is formed from "some" (from Old English *sum*, meaning a certain amount or unspecified) and "day" (from Old English *dæg*, meaning a period of 24 hours). Together, these parts convey the idea of an unspecified day in the future.
Think of an unspecified amount of time ('some') until a particular 'day' arrives — that's what someday means.