May 12 was chosen for the Celebration of Women in Mathematics because it is the birthdate of Maryam Mirzakhani. The initiative was  proposed by the Women's Committee of the Iranian Mathematical Society and voted by a vast majority of attendees to (WM)², the World Meeting for Women on Mathematics on last July 31 in Rio.

Several suggestions for local activities around May 12 are listed here.   May12 Initiative recommands free screening of the award-winning film "Women in science in Africa, A SILENT REVOLUTION" available (in english or in french) online here : womeninscienceinafrica.com/film. Please feel free to organize one and decalre it on May 12 website.

The initiative is  supported by several organisations for women in mathematics worldwide (European Women in Mathematics, Association for Women in Mathematics, African Women in Mathematics Association, Indian Women and Mathematics,  CGD-UMALCA (Comisión de Género y Diversidad de Unión Matemática de América Latina y el Caribe) and the Women's Committee of the Iranian Mathematical Society).

More information here.