I did not know until today that the first national "woman's day" was celebrated in the United States in 1909. I did remember that in 1975, the United Nations declared March 8 to be International Women's Day.
The day has historical connections to socialism, the peace movement and woman suffrage in many countries. For more info, see here, e.g., and Temma Kaplan, On the Socialist Origins of International Women's Day, 11 Feminist Stud. 163 (1985) and Leila J. Rupp, Worlds of Women: The Makings of an International Women's Movement (Princeton U. Press 1995).
Ah, to see a reference to "socialism" that is not ideologically tendentious or purely pejorative: much appreciated.