Wren Day

Wren Day (Ireland and the Isle of Man)

Public holidays in the Netherlands

The second day of the Twelve Days of Christmas (Western Christianity)
Second day of Christmas (Public holiday in the Netherlands, Poland and Slovakia)

Junkanoo

The first day of Junkanoo street parade, the second day is on the New Year's Day (The Bahamas)

Kwanzaa

The first day of Kwanzaa, celebrated until January 1 (United States)

Father's Day

Saint Stephen's Day (public holiday in Alsace, Austria, Catalonia, Croatia, the Czech Republic, Germany, Hong Kong, Italy, Ireland, Luxembourg, Poland, Slovakia and Switzerland), and its related observances:
Father's Day (Bulgaria)

Mummer's Day

Mummer's Day (Padstow, Cornwall)

Mauro Hamza

Mauro Hamza Day (Houston, Texas)

Independence and Unity Day (Slovenia)

Independence and Unity Day (Slovenia)

December 26 (Eastern Orthodox liturgics)

Christian feast day:
December 26 (Eastern Orthodox liturgics)

Vicenta María López i Vicuña

Christian feast day:
Vicenta María López i Vicuña

Synaxis

Christian feast day:
Synaxis of the Theotokos (Eastern Orthodox Church)

Saint Stephen

Christian feast day:
Saint Stephen (Western Church)

James, brother of Jesus

Christian feast day:
James the Just (Eastern Orthodox Church)

Holy Family

Christian feast day:
Earliest day on which Feast of the Holy Family can fall, celebrated on Sunday after Christmas or 30 if Christmas falls on a Sunday.[citation needed]

Pope Dionysius

Christian feast day:
Pope Dionysius