The long-standing tradition of Mother's Day goes longer back than perhaps many of us realize - riding a somewhat difficult road before it was established as a national holiday in the US (among other countries) by President Woodrow Wilson, in 1914.
Back in the Civil War, and during Reconstruction, Julia Ward Howe led an anti-war movement focusing on honoring mothers, even composing a "Mother's Day Proclamation" in 1872. However, the contemporary holiday is founded in a movement by West Virginia's Anna Jarvis, who continued her mother's efforts.
The first successful efforts came in 1908, through national promotion. After 4 years of success, Jarvis declared a stable "date" of the "second Sunday in May" for Mother's Day (Note: the punctuation is VERY important - possessive of the singular!). She initially intended the holiday to be commercial. But, ultimately, resented its overcommercialization - even being thrown in jail, 1948, for protesting the very day she founded!
White flowers are worn, or given, to honor mothers. Most notably carnations.
Apparently, according to these articles from the Bourbon News & Mountain Advocate, it doesn't REALLY matter; the flowers can be snowdrops, as long as they are white. (Personally, I remember giving my mother marigolds potted in detergent covers, when I was in elementary school. But what do I know?)
Of course, it didn't take long to call out for a day honoring the family patriarch!
(Sidebar: A movement had already begun in Washington at the time of this publication - Hartford Herald, 22 May 1912. However, Father's Day also saw trials & tribulations - including failed presidential recommendations & suggested "dates," before President Nixon proclaimed it a national holiday in 1972.)