Are Catholics allowed to celebrate the Reformation? This is going to be published in my local Catholic parish's bulletin this week.

A SPECIAL EVENT: REFORMATION 500 COMMEMORATION
Wednesday, April 26 @ 7:00pm
In honor of the 500th Anniversary of the Reformation,
St. John’s Lutheran Church will host an ecumenical vesper service. Congregations in Evansville will gather to embrace our commonalities. Ministers from the various congregations will be discussing our similarities and differences regarding Holy Baptism and Holy Communion in a relaxed atmosphere, followed by refreshments. This event is in collaboration with our partner congregations in Evansville. The whole community is invited; invite your friends and neighbors of all faith traditions!

Has anything changed in the last 500 years that Catholics should be commemorating the Reformation?
For some odd reason I thought we were against that...
Merriam-Webster define commemorate as:


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

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

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 &quot;in honor of the 500th Anniversary&quot;; I might replace it with &quot;in memory of the 500th Anniversary,&quot; 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:


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.