I really like this concept, it reminds me a bit of a combination of Helliconia and endless legend, though those are both season rather than day cycle based.

[quote]I'd say maybe a bit less emphasis on the idea of growing giant mushrooms at night, though, since hunger is definitely meant to be a significant factor in surviving the nights, and while a sensible farmer would probably try to farm things that grow in low light I'm probably going to handwave the farmers into being slightly less sensible.[/quote]
the mushroom-idea could work quite will as an initial easing into the night so that the lack of food production problem scales at a similar rate to the monster situation(also, perhaps by coincidence, the monsters become less edible as the night goes on). There is after all only a limited amount of decaying material (either natural or food that has rotten since it's harvest) around and once that is used up they have to rely entirely on food stores. 

also a few questions: 

does the planet have a moon(if so how big?) or are the stars the only source of skylight at night?

what sort of technology level would the more advanced civilizations be at military technology wise? We talking plate mail armored knights and crossbows, roman legionaries or something in between?

[quote]heroes of days long past have begun to rise once more from their graves[/quote]
Are the heroes undead looking as a result of this?