Many countries around the world celebrate National Day on fixed date, however, there are some exceptions. For instance, King's Birthday is the National Day in Thailand and it's annually celebrated on December 5. After ascension of the next king the date of the holiday will be changed.
There are several days of military honor in Russia, and one of them falls on December 5, that is Battle of Moscow Day. Commemoration of this day is connected with the events of October-January 1941-1942, when the Soviet troops defended and then launched the counteroffensive against Nazi Germany.
Day of the Ninja is a jolly and unusual holiday, that is celebrated on December 5. This holiday was created in 2003 by a parody website Ninja Burger as a day, when everyone can dress up and act as a real ninja.
International Volunteer Day for Economic and Social Development is observed on the international level every year on December 5. This day was designated by the United Nations General Assembly in 1985 to encourage spreading the information about volunteering for economic and social development.
World Soil Day is observed around the world on December 5. This holiday was proposed by the International Union of Soil Sciences in 2002 to raise public awareness of the problems of soil and its importance for our life.