A moveable fast is any fast day

Fast Day

Fast Day was a holiday observed in some parts of the United States between 1670 and 1991."A day of public fasting and prayer", it was traditionally observed in the New England states. It had its origin in days of prayer and repentance proclaimed in the early days of the American colonies by Royal...

 that is not fixed according to the Gregorian calendar

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

, i.e. is not on the same calendar date every year. Moveable fasts, like moveable feast

Moveable feast

In Christianity, a moveable feast or movable feast is a holy day – a feast day or a fast day – whose date is not fixed to a particular day of the calendar year but moves in response to the date of Easter, the date of which varies according to a complex formula...

s, are usually determined by the date of Easter

Easter

Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

, the calendar date of which is the first Sunday after the first full moon

Full moon

Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

 following the vernal equinox. Hence, Good Friday

Good Friday

Good Friday , is a religious holiday observed primarily by Christians commemorating the crucifixion of Jesus Christ and his death at Calvary. The holiday is observed during Holy Week as part of the Paschal Triduum on the Friday preceding Easter Sunday, and may coincide with the Jewish observance of...

 is always the Friday just before Easter, and Ash Wednesday

Ash Wednesday

Ash Wednesday, in the calendar of Western Christianity, is the first day of Lent and occurs 46 days before Easter. It is a moveable fast, falling on a different date each year because it is dependent on the date of Easter...

is 46 days before Easter.