Quote from Wikipedia: "The first IWD was observed on 28 February 1909 in the United States..." :)
The holiday may be a remnant from socialism indeed, but since there is no tradition of celebrating Mother's Day on a different day in the year, it's more natural to keep this holiday, instead of trying to adopt another one. And it's celebrated in a number of European countries actually.

http://en.wikipedia.org/wiki/International_Women's_Day