For those outside of Israel the requirement is for two days of yom tov (and sometimes Rosh Chodesh), originally due to uncertainty and now due to tradition. But we don't do this for all holidays even ...
Many people eat Kreplach on Purim, Erev Yom Kippur, and Hoshanah Rabah. These 3 days are considered like hidden Yomim Tovim, as work is technically allowed. They are also days that we Klap. On Purim ...