Each of these saints has his own feast day: St. Basil the Great, January 1; St. Gregory the Theologian, January 25; and St. John Chrysostom, January 27. This combined feast day, January 30, was instituted in the eleventh century during the reign of Emperor Alexius Comnenus. Once, a debate arose among the people concerning who among the three was the greatest. Some extolled Basil because of his purity and courage; others extolled Gregory for his unequalled depth and lofty mind in theology; still others extolled Chrysostom because of his eloquence and clarity in expounding the Faith. Thus some were called Basilians, others Gregorians, and the third were called Johannites. This debate was settled by divine providence, to the benefit of the Church and to the even greater glory of the three saints. Bishop John of Euchaita (June 14) had a vision in a dream: At first all three of these saints appeared to him separately in great glory and indescribable beauty, and after that, all three appeared together. They said to him: “As you see, we are one in God and there is nothing contradictory in us; neither is there a first or a second among us.” The saints also advised Bishop John to write a common service for them and to order a common feast day of celebration. Following this wonderful vision, the debate was settled in this manner: January 30 would be designated as the common feast of these three hierarchs. The Greeks consider the feast not only an ecclesiastical feast, but also their greatest national and school holiday.
Source: St. Nikolai Velimirovic, The Prologue of Ohrid – Volume One.
The three most great luminaries of the Three-Sun Divinity have illumined all of the world with the rays of doctrines divine and true; they are the sweetly-flowing rivers of wisdom, who with godly knowledge have watered all creation in clear and mighty streams: The great and sacred Basil, and the Theologian, wise Gregory, together with the renowned John, the famed Chrysostom of golden speech. Let us all who love their divinely-wise words come together, honouring them with hymns; for ceaselessly they offer entreaty for us to the Trinity.