Accordingly, many Protestants--principally Lutherans, but also Presbyterians and other Reformed denominations--celebrate October 31 as Reformation Day. For convenience, local churches often transfer the celebration to the preceding Sunday, hence the name Reformation Sunday.
In earlier days, when there was a great deal more animosity between Catholics and Protestants, the latter group often celebrated Reformation Day as we would celebrate a military victory. There would be triumphal processions, hymns glorifying the church, and often the sermon would be a diatribe against the "Papists." (To be fair, during the period the Catholics--in a different part of Europe--would be dealing the venom right back to the "heretics.")
Today, while the hymns remain (as they should!) many Protestant and Catholic churches use Reformation Sunday as a opportunity to highlight areas of common agreement. On Oct 31, 1999, the Lutheran World Federation (of which the ELCA is a member) and the Roman Catholic Church signed a joint declaration on the doctrine of justification, clarifying positions where Lutherans and Catholics agree and disagree.