Mothers have been around forever, of course - I mean, we literally wouldn't be here if it weren't for them, but Mothers' Day itself is a long-standing tradition with roots going back to medieval times. Originally, it was the day when people visited the "Mother Church" and, in the UK, it was always the 4th Sunday in Lent. It wasn't until the 17th century that it became a day for acknowledging our own mothers.
Mothers Day is still the 4th Sunday in Lent in England, so after I moved away, first to Germany then to the US, I often forgot to send my own mother a card until May. (Mothers Day is the 2nd Sunday in May in Germany too, just like the US.) She still loved me though, after all, she was my "mum" and loved me unconditionally. For the most part I'd always remember to buy her a card in May then save it until the following Spring. I'd send her one in May too. I think she liked getting more than one card at different times of the year.
I always loved my mum and knew without a shadow of a doubt that she loved me too, but it wasn't until I became a mother myself that I fully realized the depth and overwhelming nature of that special love between a mother and her children. My mum passed away in 2001, but she will be in my heart forever.
I love you mum!