Are Catholics allowed to celebrate the Reformation?

Canon 64 of the Apostolic Canons:

If any clergyman or layman shall enter into a synagogue of Jews or heretics to pray, let the former be deposed and let the latter be excommunicated.

I believe that these Canons still have force within the Roman Catholic Church (the link is to a Catholic web site), but I cannot find them on the Vatican web site.

I suppose if no one prays at the event, it would be ok for Roman Catholics (and Orthodox) to attend.

It seems that Catholics are permitted to commemorate the 500 anniversary since the Reformation. Although the Catholic Church may not agree with many things that the Reformation brought about, it is an excellent opportunity for the Church to promote Christian unity. Ut Unum Sint 'That they may be one'. Praying for Christian unity is most commendable, even for us Catholics.

Pope Francis himself was at the ecumenical events in Lund and Malmö, in Sweden, on the 31st of October, 2016 as a sign for the need of Christian unity.

Pope Francis, LWF President Bishop Dr Munib A. Younan and LWF General Secretary Rev. Dr Martin Junge will lead the common prayer service in Lund and the event in Malmö in cooperation with leaders from Church of Sweden and the Catholic Diocese of Stockholm.

“There is power when communities find their way out of conflict. In Christ we are encouraged to serve together in this world. The joint commemoration is a witness to the love and hope we all have because of the grace of God,” LWF President Younan and General Secretary Junge say.

Kurt Cardinal Koch, President of the Pontifical Council for Promoting Christian Unity explains further: “By concentrating together on the centrality of the question of God and on a Christocentric approach, Lutherans and Catholics will have the possibility of an ecumenical commemoration of the Reformation, not simply in a pragmatic way, but in the deep sense of faith in the crucified and resurrected Christ.”

Bishop Anders Arborelius of the Catholic Diocese of Stockholm adds, “History will be written when Pope Francis and the LWF leaders visit Lund and Malmö to encourage all of us to go further on the road towards Christian unity.” - Preparations to commemorate 500 years since the Reformation, 01.06.2016 (Vatican Website).

Kurt Cardinal Koch (Pontifical Council for Promoting Christian Unity), Bishop Dr. Munib Younan (LWF President), Pope Franics and Rev. Dr. Martin Junge, lead the Common Prayer in Lund Cathedral on 31 October 2016. Photo: Church of Sweden/Magnus Aronson

No.

Protestants hold several heresies; e.g., they deny

• the ministerial priesthood

• the sacrificial nature of the Mass

• the necessity of good works and not faith alone for salvation

• the papacy

• the Immaculate Conception

• etc. (see this: "Heresy in Protestantism")

If a Catholic pertinaciously celebrates / assents to heresy, he at best commits a mortal sin and at worst is also excommunicated (cf. this).

cf. Facts about Luther by Msgr. Patrick F. O'Hare, LL.D.

Merriam-Webster define commemorate as:

1. to call to remembrance: St. Andrew is commemorated on November 30.

2. to mark by some ceremony or observation; observe: commemorate an anniversary

3. to serve as a memorial of: a plaque that commemorates the battle

In this sense, we cannot be opposed to commemorating (particularly in sense 1) the Reformation, lest we forget its causes and allow it to happen again.

As a Catholic, though, I would be opposed to leaving "in honor of the 500th Anniversary"; I might replace it with "in memory of the 500th Anniversary," since, as others have pointed out, we cannot honor a wound to the unity of the Church, however much Luther and Calvin might have thought it necessary.

That being said, ecumenical dialogue is something required of the Church by virtue of it being Church:

1. The restoration of unity among all Christians is one of the principal concerns of the Second Vatican Council. Christ the Lord founded one Church and one Church only. However, many Christian communions present themselves to men as the true inheritors of Jesus Christ; all indeed profess to be followers of the Lord but differ in mind and go their different ways, as if Christ Himself were divided.^{Cf. I Cor 1:13} Such division openly contradicts the will of Christ, scandalizes the world, and damages the holy cause of preaching the Gospel to every creature. — Unitatis Redintegratio

So, in a sense, we as Church are required to attend such events and go seek restoration of the unity that Christ preached, because without our cooperation, it will not fall down on our laps.