Moveable fast

A moveable fast is any fast day that is not fixed according to the Gregorian calendar, i.e. is not on the same calendar date every year. Moveable fasts, like moveable feasts, are usually determined by the date of Easter, the calendar date of which is the first Sunday after the first full moon following the vernal equinox. Hence, Good Friday is always the Friday just before Easter, and Ash Wednesday is 46 days before Easter.