In the Reformation tradition, today is referred to as "Reformation Sunday." It is the Sunday in October closest to October 31, when Martin Luther nailed his 95 theses to the Wittenberg door (in the photo at right) in 1517. Some Protestants treat Reformation Sunday as a day to be celebrated. That is understandable if one conceives of Luther as a 16th century Moses who led God's chosen people out of the "Babylonian Captivity" of the Church into the promised land of Protestantism. But there is another way to conceive Reformation Sunday, even as a Protestant. In this other way, while there were indeed abuses in the Church that needed correcting, the Protestant schism was a tragedy with terrible effects on the world, especially in Europe, as many people lost their faith in God altogether on account of religious wars and all sorts of incompatible interpretations of Scripture. Indifferentism, liberalism and rationalism filled the authority vacuum. This schism to this day prevents Protestants and Catholics from sharing the same Eucharist, which is that sacrament by which we are one Body. (1 Cor 10:17) For these reasons, the other perspective sees Reformation Sunday as a day of mourning, not rejoicing. Celebrating an event causes the celebrants to overlook what was harmful about the event. The danger of celebrating Reformation Sunday is that one can, at the same time, even without intention, end up celebrating schism, when instead we should be mourning this schism, so as to remind ourselves of the daily need to be working to resolve this schism.
Stanley Hauerwas helps us see Reformation Sunday from that other perspective. His article can be found here.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)