Every day since, I have spotted other shops who agree with Easons.
I had understood that today was a day for all mothers and so assumed it was “Mothers’ Day” – but seemingly not.
Wikipedia advises that it was intended the day should be for each family to celebrate its own mother and so ‘Mother’s Day’.
But it still appears grammatically incorrect to me.
‘In 1912, Anna Jarvis trademarked the phrases "second Sunday in May" and "Mother's Day", and created the Mother's Day International Association. She specifically noted that "Mother's" should "be a singular possessive, for each family to honor its mother, not a plural possessive commemorating all mothers of the world."’