There was a time not many years ago when the concept of “mother” was for the most part clear-cut. Your mother was the female person who gave birth to you and who nurtured you.
In that context, the context within which Mothers Day was observed was also pretty straightforward— see the above paragraph.
On this topic as much as any other, society has in recent years changed. We now have bio moms, egg donor moms, grandparent moms, same-sex partner moms (including some males) and surrogate moms, not to mention step moms. There are now so many varieties of moms that society has coined a term for them: othermoms.
There are still also regular moms, by the way, which, all things being equal, probably remains the preferred option.
All of this means that for better or for worse, what is actually being celebrated on Mothers Day — beyond commercialization — is more of a conceptual expression of the duties and execution of motherhood than the role itself. In 2013, society is not shunting aside the biological claimant to the title, but it is broadening that recognition to encompass the emotional and practical claimants as well. These are less frequently one and the same.
Whether this adjustment in the definition of motherhood is good or bad for society is, to a great extent, beside the point. The definitional change is happening, and the sociologically safest prediction is that it will only continue to happen. Modern society utterly adores the flexibility provided by othermom.
So Happy Mothers Day, mom…in all your numerous varieties.