882. How to Act on a Personal Fast

127:4 Even if one did not commit to fast by saying so aloud, but rather he thought about it and decided to fast on the next day, even if this was not during mincha but before or after mincha so long as it was still day, then it is still considered a commitment and he is required to fast.

127:5 One who is fasting should not indulge himself, nor should he act frivolously or joyfully. Rather, he should act concerned and mournful as per Eicha 3:39, "Of what should a living man be concerned? A man for his sins."