Cautionary tale...the definition of "don't assume, it makes an ass out of you and me". When I moved to the US, I assumed that mother's day (like every other shared celebration) was on the same date on both sides of the Atlantic.  I realized my mistake when, UK mother's day having passed by in a state of silence, I received parental accusations of the "mind like a sieve" variety. Claims that the big MD was the following month...and that festive plans were in place...fell on deaf ears (until corroborating evidence could be supplied).

The moral of the story...celebrate mother's day early, and often...and never assume...