Today is the feast day of Saint David, so a simple explanation would be that it's like St. Patrick's Day, but for the Welsh!
For those of us into Shakespeare, In Henry V, after the victory at Agincourt...
Captain Fluellen: 'I do believe your Majesty takes no scorn to wear the leek upon Saint Davy's day. '
King Henry: [The king laughs through his tears] 'I wear it for a memorable honour. For I am Welsh, you know'.
The Leek, is another word for the daffodil flower as it is known in English. SO..., IF you be Welsh, then wear a daffodil today, as a buttoneer
GOD SAVE WALES
LD