Most common cause of convulsion on the First day of life in a newborn is –
Neonatal seizures

The most prominent feature of neurological dysfunction in the neonatal period are seizures.
The five major causes of neonatal seizures are -


Hypoxic ischemic enceplialopathy (hypoxia, anoxia) → most common       
Hypocalcemia 
Hypoglycemia
Meningitis
Polycythemia
Anoxia
Head Injury
Hypoglycemia
Hypocalcemia