I thought that it was something to do with Robert Recorde, who invented the = sign .... but he came from Tenby, so it can't be - or can it?
No, it's not, as this is what Google says about one of the village's famous inhabitants:
"Welsh mathematician William Jones (1675 - 3 July 1749) coined the symbol π ( 'pai' in Welsh; the Greek letter pi ) to represent the ratio between the circumference of a circle and its diameter.
He was born in the parish and was raised in Llanfihangel Tre'r Poets, on Anglesey before moving to Llanbabo, and was educated at Ysgol Llanfechell."
I'm not sure what the significance of the date of 14th March is, though - was it the date on which he made his discovery?