Someday and one day
are implicitly dissimilar.

Because
"Someday I'll find love"
is full of ambivalent hope.
But
"One day I'll find love"
expresses certainty.

Because
"Someday I'll go on an adventure"
really means "I sure hope I get to travel."
But
"One day I'll go on an adventure"
implies "I will leave this town."

Because
"Someday I'll love someone else"
means "I'm not over my ex."
But
"One day I'll love someone else"
means "On to the next chapter."

Someday and one day
are commonly mistaken as equal.
But someday won't come,
while one day is just around the corner.