Frequent misunderstandings arise concerning the purpose, meaning and application of the Protestant Reformation. It's such a multifaceted epoch in history that widely varied opinions and interpretations exist regarding whether it was good or bad. The Reformation of the 16th Century is at once both a tragic tale, and a glorious one. It's a tragic one because an unintended consequence of it was the subsequent splintering of the Church of the West. However, it's a marvelous story because the need for the western Church to correct some of its errors was THAT dire. Basically, it's both a fond and painful memory for all of us - not unlike how an adult recalls a particular meaningful spanking they received as a child. For this reason, some may cringe at the prospect of celebrating the Protestant Reformation on October 31st (the traditional date in 1517 when Martin Luther nailed his "95 Theses" to the front door of his church in Wittenburg), and understandably so. On the other hand, I believe it's deserving of a commemorative party - not unlike how adults recall a younger spanking with stories of childhood discipline that resemble, "Yeah. My father was a loving, but strict man. I remember one time I acted up...and instead of just leaving me to my self-destructive folly, he applied the 'board of education' to the 'seat of learning.' I'm so glad he loved me enough to straighten me up then...but I still couldn't sit down comfortably for days."
To the Philippians, the Apostle Paul wrote, "For I am sure of this very thing, that the one who began a good work in you will perfect it until the day of Christ Jesus" (Phil 1:6; emphasis added). This confident statement by the Apostle can be appropriately interpreted as Divine assurance that God will remain an "engaged Father," disciplining and correcting the Church (semper reformanda - "always reforming") until the return of Jesus Christ. Far from standing aloof, allowing the Bride of Christ to wander aimlessly in error, he set events in motion and supervises them to fix things that are broken. There can be no doubt that the disunity that ensued has left us rubbing our collective backsides, saying "Oooh! That smarts." Nevertheless, the bruised posterior offers both bragging rights for the Herculean endurance and an endearing narrative concerning an attentive Father.
Therefore, while I understand that reluctance to celebrate the Reformation can be born of a legitimate distaste for the division in the Church that has sense ensued, I instead take the position of recalling the episode as a story of God's grace. Imagine the opposite scenario of a disengaged father that leaves his offspring to their own devices, meandering about without corrective intervention. Surely the the healing process continues, and we will one day see a Church re-unified in it's doctrine and mission. In the meantime, celebrating Reformation Day is to celebrate the God that corrects us when we need it. He remains interactive and engaged, bringing us back on track when we wander off it (i.e. indulgences, Transubstantiation, etc.).
Some have exploited this discipline though, using the post-Reformation division to advance their own sect, seemingly reveling in the tragic pain of a splintered church. They exult in the division and assume that efforts to heal from these wounds and seek ecumenical unity are misguided. This is a wrong application of the Reformation "spirit," by seemingly wanting to keep the "spanking" going on longer than is necessary. On the contrary, as any loving Father holds and consoles the very same child they just painfully disciplined, so also would the Father like to see us united as "one, holy, catholic and Apostolic Church" again. Until that oneness is again realized (and even when it is), celebrating the God who fixes us is appropriate for any time.
In addition, Reformation Day is a celebration of the Church about the Church. It was the Church's very process of theological reflection and geographic expansion that God used to correct the errors of the medieval era. Scholars such as Martin Luther wrote on the abuses of the Roman church, calling for their correction, desiring reform more than discord. Thomas Cranmer and others advanced Christian worship free of odd innovations to classic Christian teaching that arose from Rome as well. All of these events reinforce that when God wanted his Church back in the right path, he used the Church to do it. So Reformation Day commemorates the triumph of the Church in addition to celebrating the triumph of God and his Word.
All of this suggests the necessity for a grand party, feast, celebration for God's people. It's a legitimate reason to gather the Lord's people together and incite them to revel in God's goodness demonstrated in "fixing" us when needed. When we celebrate our "engaged Father" that has reformed us before, we too can be "sure of this very thing, that the one who began a good work in you [us] will perfect it until the day of Christ Jesus." As a result, his work of semper reformanda ("always reforming") in us is his loving attention to the Church to keep us faithful to him and his mission to the world.