Reformation Day is a religious holiday celebrated on October 31 in remembrance of the Reformation, particularly by Lutheran and some Reformed church communities. It is a civic holiday in Slovenia (since the Reformation contributed to its cultural development profoundly, although Slovenes are mainly Roman Catholics) and in the German states of Brandenburg, Mecklenburg-Vorpommern, Saxony, Saxony-Anhalt, and Thuringia. It is also a national holiday in Chile since 2008.

In the United States churches often transfer the holiday, so that it falls on the Sunday (called Reformation Sunday) on or before October 31, with All Saints' Day moved to the Sunday on or after November 1.^{[citation needed]}