Mother's Day is a celebration honoring mothers and celebrating motherhood, maternal bonds, and the influence of mothers in society.
Mothers are special all year round but there is one day a year dedicated to the celebration of mothers.  This celebration is recognized all over the world, although, not every country celebrates it at the same time.  However, the most common months Mothers day it is celebrated around the world is March, April or May.
Birth moms are the most common but it is perfectly appropriate to honor a very good friend that a strong maternal bond was formulated during a childs life as their mother.  It is also acceptable to honor Grandmother's as well.
In a way you could say it is a second birthday celebration each year for us as we are celebrating our birth with the honoring of the woman giving birth to us or the finding of a maternal bond with another woman.
We all have those ups and downs in relationships but Mothers Day is the time to set aside differences and celebrate.  
Mother's Day gift baskets are a great way to express your feelings whether you deliver the basket yourself or are needing to have it shipped to that special mom.