Whether the joy of Yom Tov is so important that we overlook certain halachic considerations in order to ensure that one will create this joy: Beitzah 7b, 8a, 9b-10a, 11a-b
Exercising leniency regarding laws of impurity, in order to ensure that people will bake ample bread and open good wine for Yom Tov: Beitzah 11b