As I understand it, Lutherans and Calvinists are the chief observers of Reformation Day, October 31st. Many churches in the US opt for a weekend celebration, and move the feast to the last Sunday of October. Today is the earliest it can occur.
I know there have been gestures from Roman Catholicism in the direction of this commemoration of Martin Luther’s gauntlet from 503 years ago. But it’s mostly ignored in places I’ve served.
However, assuming your parish is aware of it, how would you think an observance in a Catholic community should unfold? Consulting with the Protestants down the street so as not to blunder into an ecumenical faux pas? Offer a prayer for Christian unity during the intercessions? Celebrate a choir event (in non-pandemic times)? Sing German hymn tunes at Mass?
On that last piece, I once lived in a town where the churches did gather for a choir festival on a late October evening. I don’t recall we sang much along the lines of Ein Feste Berg. But it did have a following of sorts, if primarily along the lines of singers’ spouses.
Any thoughts from the purple chair?
Catholic Sensibility 
I don’t believe it should be observed liturgically as such in a Catholic church on a Sunday, any more than I’d expect the local Presbyterian church to celebrate a variety of distinctively Catholic feasts. There were many reformations, including a variety of Catholic ones, and the Lutheran one is really proper to Lutherans and its progeny.
A votive Mass for the Unity of Christians could be celebrated on unimpeded ferias during the week, if there’s a desire for a liturgical manifestation of related concern.
One could also have a service of communal reconciliation with a specific mission/focus of sins against the unity of Christian sisterhood and brotherhood.