The one-word adverb someday works when describing an indefinite future time (e.g., “I’d like to see him again someday”). Some day is two words when it refers to a single day, even if that day is unknown or not specified (e.g., “I have an appointment some day next month”).
The distinction is useful, but despite its usefulness and in spite of what usage authorities say, many writers use someday and some day more or less interchangeably.
She also sees the sunflower as someday becoming the official symbol of hope for those suffering from the disease. [Cincinnati.com]
It is possible that, someday, the schools, libraries, fire stations and park pavilions built in 2010 will be seen as the best and most carefully designed of the decade. [Wall Street Journal]