Just as in the United States, Mother's Day or Muttertag, is celebrated on the Second Sunday of May. In Germany, that rule only changes when the date conflicts with Pentecost. Pentecost a Christian holiday that celebrates the descent of the Holy Spirit on Jesus's followers. It is a moveable holiday and is celebrated 50 days after Easter.
Mother's Day has been a national holiday in the United States since 1914, when it was declared by then President Woodrow Wilson. It was first celebrated in 1908 by Anna Jarvis. It was in 1912 declared a holiday by the West Virginia Governor and then Pennsylvania followed the year later. 1914 it became everyone's holiday.
It was not a holiday in Germany until the year 1922. It was then when Mother's were celebrated for the children they produced for Germany.
It is celebrated in Germany just as it is in the United States...candy, cards, flowers and very crowded restaurants. You often find German families celebrating with family cook-outs or BBQ's.