As to who St David was and what he did, it's probably best to refer you simply to Wikipedia or Google rather than cut and paste huge amounts of information.
Most importantly for this thread (and the other, above the line), St David is the patron saint of Wales.   So for St David's Day it's a great chance to celebrate Wales and the Welsh. When I was at school we had an Eisteddfod which lasted an afternoon and the following morning and then had a half day holiday after that - the Eisteddfod included competitions for all school subjects but chiefly of course poetry and music. These days there isn't a half day holiday (see the other thread for Dr Price's point of view, which I agree with!) but there are all sorts of other events.   There's a great parade in Cardiff, there are concerts and parties, we had a day on Saturday handing out daffodils, Welsh cakes, sweets, stories and songs in the local market and a day on Sunday at the leisure centre with face painting, dancing, a parade, singing ....lots of stuff. People wear daffodils or leeks as corsages, dress in Welsh costume if they feel so inclined (or just put on a rugby shirt) and have a good time.

Come to Wales! Find out at first hand!

Anne