Over 70 other countries celebrate it on today as well. Besides the US, also Canada, India, China, South Africa and a large part of Europe is in this list; represents a decent number of HN'ers here, I thought =)
Yes, but in many of those countries it's already Monday.
Since my mother lives nineteen timezones ahead of me I have to be pretty on top of the whole "mother's day" thing.
I happen to know for a fact (because I was just there for a week), but today is Mother's Day in Japan too.