* In the Roman Catholic calendar, May 1 is the feast of St. Joseph the worker.
* In the neopagan Wheel of the Year May begins on Beltane in the northern hemisphere and Samhain in the southern hemisphere.
* In the Irish calendar May 1 is Beltane (Bealtaine), the first day of Summer, and a public holiday is held on the first Monday in May.
* In many countries, May Day is May 1. This is celebrated as Labor Day in many countries.
* In the United Kingdom, May Day is May 1, but a public holiday is held on the first Monday in May.
* In Germany it is an old custom to plant a "tree of May" to honor someone. Often young men set up an adorned birch in front of their girlfriend's house in the night before May 1.
* In Japan, there is a so-called 'May sickness', where new students or workers start to be tired of their new schoolwork or jobs. (In Japan schoolyears and fiscal years start on April 1st.)
* The first Saturday in May is the date of the annual Kentucky Derby, the most famous horse race in the United States.
* May 4th in the Netherlands is the day of Remembrance of the Dead, commemorating all the casualties in military conflicts involving the Netherlands.
* Cinco de Mayo or the Batalla de Puebla is celebrated in Mexico on May 5. It is also celebrated in the United States wherever there is a sizable Mexican community.
* May 5th is the Children's Day in Japan, Korea and China
* May 5th is Liberation Day in the Netherlands, celebrating the ending of the German occupation.
* In the Europe May 5 is Europe Day (uncommon usage, largely replaced by May 9)
* In Western Europe May 8 is VE Day. In Eastern Europe it is celebrated on May 9.
* In the European Union May 9 is Europe Day
* Golden Spike Day (1869 - Completion of the First Transcontinental Railroad - Promontory Summit, Utah)
* On May 12, International Nurses Day is celebrated.
* Finland's Day in Finland on the 12th of May.
* May 12 is also International Awareness Day for Chronic Immunological and Neurological Diseases (CIND). These diseases include Neurofibromatosis, Chronic Fatigue Immune Dysfunction Syndrome (CFIDS)/Myalgic Encephalomyelitis, Fibromyalgia, Gulf War Syndrome and Multiple Chemical Sensitivity.
* On May 17, the Norwegian Constitution Day is celebrated.
* In Chile, the Battle of Iquique (Combate Naval de Iquique), is celebrated on May 21, and it is a national holiday.
* Eritrea celebrates its Independence Day (Independence from Ethiopia) on May 24.
* In Ecuador, May 24 is remembered and celebrated as the day of the Battle of Pichincha - May 24, 1823.
* In Argentina, the May Revolution (or Revolución de Mayo), a national holiday, is on May 25.
* Towel Day is celebrated in May in tribute to Douglas Adams, author of The Hitchhiker's Guide to the Galaxy.
* Under the French Ancien Régime, it was of habit to "plant a May" or a "tree of May" in the honor of somebody. The County of Nice saw girls and boys "turn the May" with the sound of fife and drum, i.e. to dance rounds of May around the tree of May planted on the place of the village.
* The second Sunday in May is Mother's Day in the United States.
* Each year in May, the Eurovision Song Contest is held.
* The Indianapolis 500 is held on the Sunday before Memorial Day.
* Labor Day in Queensland, Australia is celebrated on the 1st Monday in May.
* In Canada, Victoria Day is celebrated on the last Monday on or before May 24.