Mothers and motherhood have been honored in many different ways for thousands of years. An example is the ancient Greeks cults to Cybele and Rhea. 
However, the known modern holiday began in the United States in 1908. 

Anna Jarvis held a service at her West Virginia church to honor her late mother that began her campaign to create a national holiday. 

Congress rejected a public holiday that year, but by 1911, all fifty states had recognized it and in 1914 President Wilson proclaimed that the second Sunday in May should be celebrated as Mother’s Day.  

It still celebrated on the day a century later. 

 Jarvis insisted the holiday be spelled with the singular possessive because she believed each family should honor their mother as a person not the concept of motherhood.