It is also like ascension or easter or pfingsten, all these christian holidays which are derived at how the moon has been standing at a particular time of year xD , So this year that is pretty late .
In Germany and Southafrica it is also in the second sunday of May btw.
Mothers day was invented by a woman who thougght there should be a special day for all mothers (d'uh) , It was on the sunday after the day the second year that her mother died. father day in Germany is the same day as the ascension day is.(40 days after easter)