From 326 A.D. (the first year following the Council), Pascha has been officially defined as the first Sunday following the date of the Paschal Full Moon ("PFM") for a given year. The PFM is not, however, as commonly thought, the first full moon following the vernal equinox. Rather, the PFM is the first Ecclesiastical Full Moon ("EFM") date that follows March 20. EFMs are calendar dates that approximate the cycle of astronomical full moons (usually falling within 1-3 days of an astronomical full moon), which repeats every 19 years. March 20 is the date used for determining the PFM because it was the vernal equinox in 325 A.D., the year the EFM cycle was determined by astronomers.  Because of the inaccuracy of the [[Julian Calendar]], Pascha is drifting later into the year for those who use the Julian Paschalion. Thus, while according to the calendar, for those using the Julian Calendar Pascha will always be sometime in March or April, it will eventually be celebrated in the northern hemisphere in the summer, the autumn, and then the winter. For those using the [[Revised Julian Calendar]], the calendar date of Pascha is drifting along with its astronomical position. The [[Gregorian Calendar]], which includes its own revised Paschalion, has neither of these problems.