Beltane

May Day (beginning of Summer) observances in the Northern hemisphere (see April 30):
Beltane (Gaelic)

Calan Mai

May Day (beginning of Summer) observances in the Northern hemisphere (see April 30):
Calan Mai (Wales)

Loyalty Day

International Workers' Day or Labour Day (International), and its related observances:
Loyalty Day, formerly intended to counterbalance the celebration of Labour Day. (United States)

Law Day (United States)

International Workers' Day or Labour Day (International), and its related observances:
Law Day (United States), formerly intended to counterbalance the celebration of Labour Day. (United States)

Lei Day

Lei Day (Hawaii)

Guerrilla gardening

International Sunflower Guerrilla Gardening Day

Maharashtra Day

Commemoration of the states of Maharashtra and Gujarat following the foundation of Samyukta Maharashtra Samiti (India):
Maharashtra Day

Constitution Day

Constitution Day (Argentina, Latvia, Marshall Islands)

Armed Forces Day

Armed Forces Day (Mauritania)

May 1 (Eastern Orthodox liturgics)

Christian feast day:
May 1 (Eastern Orthodox liturgics)

Saint Ultan

Christian feast day:
Ultan

Theodard

Christian feast day:
Theodard

Sigismund of Burgundy

Christian feast day:
Sigismund of Burgundy

Torquatus of Acci

Christian feast day:
Seven Apostolic Men
Torquatus of Acci

Secundus of Abula

Christian feast day:
Seven Apostolic Men
Secundus of Abula