Sefer Chasidim Siman 862
Can copy this into an AI, or connect the whole library to your assistant. For deeper learning, you can directly use the original seforim, sites, and apps.
כל ימי עני רעים אפילו בשבת ויום טוב מפני שנוי וסת וטוב לב משתה תמיד פירוש וטוב לב משתה תמיד כדי שייטיב לבו ולא יעצב:
Medieval Jewish mysticism, translated by Sholom Alchanan Singer. Northbrook, Ill. Whitehall Co., 1971
All days of a poor person are bad and even Sabbaths and festivals because he suffers from change of routine diet however "he who has a joyful heart has a continual fast" meaning that in so doing he will not come to have his heart saddened (by a changing diet) but rather his heart will be a joyful one.