A "fine day" is a term used to describe a pleasant, sunny day with good weather conditions. It is often used to express satisfaction or happiness with the current situation.
Sentences with «fine day»
On fine days, enjoy the panoramic view from the roof. (sofitel.com)
Both cats and dogs usually feel fine the day after surgery. (scenichillsvet.com)
Also you said you tried to start it and it wouldn't start the next day after it was running fine the day before. (mechanics.stackexchange.com)