First Council of Nicaea - Wikipedia

Today is a very important day in Church history. On May 20, 325, the Ecumenical council is inaugurated by Emperor Constantine in Nicea.
From the article:

"The First Council of Nicaea was the first ecumenical council of the Church.[9] Most significantly, it resulted in the first uniform Christian doctrine, called the Nicene Creed. With the creation of the creed, a precedent was established for subsequent local and regional councils of Bishops (Synods) to create statements of belief and canons of doctrinal orthodoxy—the intent being to define unity of beliefs for the whole of Christendom.[citation needed]

Derived from Greek (Ancient Greek: οἰκουμένη, translit. oikouménē, lit. 'the inhabited one'), "ecumenical" means "worldwide" but generally is assumed to be limited to the known inhabited Earth, (Danker 2000, pp. 699-670) and at this time in history is synonymous with the Roman Empire; the earliest extant uses of the term for a council are Eusebius' Life of Constantine 3.6[10] around 338, which states "he convoked an Ecumenical Council" (Ancient Greek: σύνοδον οἰκουμενικὴν συνεκρότει, translit. sýnodon oikoumenikḕn synekrótei)[11] and the Letter in 382 to Pope Damasus I and the Latin bishops from the First Council of Constantinople.[12]

One purpose of the council was to resolve disagreements arising from within the Church of Alexandria over the nature of the Son in his relationship to the Father: in particular, whether the Son had been 'begotten' by the Father from his own being, and therefore having no beginning, or else created out of nothing, and therefore having a beginning.[13] St. Alexander of Alexandria and Athanasius took the first position; the popular presbyter Arius, from whom the term Arianism comes, took the second. The council decided against the Arians overwhelmingly (of the estimated 250–318 attendees, all but two agreed to sign the creed and these two, along with Arius, were banished to Illyria).[9][14]

Another result of the council was an agreement on when to celebrate Easter, the most important feast of the ecclesiastical calendar, decreed in an epistle to the Church of Alexandria in which is simply stated:

We also send you the good news of the settlement concerning the holy pasch, namely that in answer to your prayers this question also has been resolved. All the brethren in the East who have hitherto followed the Jewish practice will henceforth observe the custom of the Romans and of yourselves and of all of us who from ancient times have kept Easter together with you.[15]

Historically significant as the first effort to attain consensus in the church through an assembly representing all of Christendom,[16] the Council was the first occasion where the technical aspects of Christology were discussed.[16] Through it a precedent was set for subsequent general councils to adopt creeds and canons. This council is generally considered the beginning of the period of the First seven Ecumenical Councils in the History of Christianity."