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.Please feel free to organize one and mention it on May 12 website.
In the countries where there are restrictions to gatherings because of the coronavirus crisis, organizing collective events with physical presence might be difficult this year. But, adapting our suggestions, it might be possible to organize events differently and virtually. .
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, Colectivo de Mujeres Matemáticas de Chile andthe Women's Committee of the Iranian Mathematical Society).
More information here.