"Someday" is more appropriate than "One day".

Question: When will you get down to writing your book?
Answer: One day.

vs.

Question: When will you get down to writing your book?
Answer: Someday.

An abbreviation of "Someday I will."

"One day" is too literal a translation from "Isang araw". It indicates that the speaker is thinking not in English but in Bahasa/Tagalog.