Shaunaka Smriti says:
"Ashtamyam cha chadurdashyam na kuryaath bhojanam dhiva
AADITYA parva sankranthau vyathipathae pitur dinae
Abithashchopavasasya na kuryan NISHI BHOJANAM"
MEANING:
During ashtami,chadurdasi dont eat during noon.During sunday,amavasya,poornima,month birth,vyathipatham,pitur dinam dont eat during night.
There r mantras for prayashchitta if u eat!
