They're for honoring the memory of William J. Card, a late 19th Century railroad magnate, known for treating his employees well and even letting hobos ride on his trains for free. He was kidnapped and held for ransom for 30 days until all the hobos who knew of his generosity pooled all of their change and raised the money to free him, but his train home was tragically derailed and he never made it. Every year since then a cherry tree has been planted in Washington DC on March 1st in his memory.

Google "Cardcaptor Sakura" to learn more.