The Council of Nicaea in 325
The most important holiday for practising Christians is Easter, a holiday with both historic and religious links to the Jewish holiday of Passover. In the second century, both of the holidays were celebrated at the same time, always on the fourteenth day of the month of Nisan, regardless of what day of the week it fell on. Controversy would soon erupt among early Christians over which day to celebrate Easter, only to be tentatively resolved when it was agreed that the holiday should be tied to the spring full moon. The agreement was short-lived, as some Christians began to deviate from the agreement and chose to celebrate the holiday on Sunday, in order to distinguish it from the Jewish holiday. In the end, two calendars were used, the Roman and the Coptic, so there was disagreement as to when the date of Easter occurred. It was not until the very first ecclesiastical council of Nicaea in 325 (the city is now known by its Turkish name of Iznik) that the Coptic method of calculation would be chosen. The bishop of Alexandria (the “Coptic” calendar is sometimes referred to as “Alexandrian”) was given the task of calculating the date and making it known. According to this calculation, Easter was to be celebrated on the Sunday following the first full week of spring, with the following spring, according to the council, to begin on the 21st of March. This took many years to realise, as the final unification of the calculation for Easter day would not take place until the end of the 8th century.
I will allow myself a little clarification here, the following sentence can be found in the professional literature: according to an exact astronomical calculation, the beginning of spring in 325 came on March 20 at 13:53 local Nicene time. This calculation comes from the epoch-making work of F. K. Ginzel, Handbuch Der Mathematischen und Technischen Chronologie, published in 1906. According to current astronomical calculations, it was at 14:01, which is quite a coincidence. The problem, however, is that in the twentieth century, it was discovered that the Earth rotated irregularly and the length of the day changed over the centuries. The result of astronomical calculations is then the so-called terrestrial time, which still needs to be converted to solar time. In retrospect, it was calculated that in 325 the difference between the two times was 2 hours and 4 minutes (however, other values are given). So, according to current calculations, the spring of 325 began in Nicaea at 11:57 local time. However, in the sense that the spring began on March 20, nothing changes, just for the sake of accuracy.