because saint brigid is a 'christian' reincarnation of celtic pagan godess, and her 'saint's day, Feb 1st, marks an important day in the celtic calendar-it is the birth of spring (you wouldn't know it here in NY!)

as for why brigid vs brigit.. its like paddy vs patty. My father, Patrick was often called Pat by those who knew him, but close friends new he was Paddy.