Complex sentence: The day he arrived in Hoenn was his eleventh birthday, and he ran away from his new home on that day, since he thought Norman wouldn't endorse his choice to be a Coordinator (Contest trainer).
Split into simpler sentences: The day he arrived in Hoenn was his eleventh birthday. On that very day, he ran away from his new home since he thought Norman would not endorse his choice to be a Coordinator (Contest trainer).