Thursday 15th March was the ancient celebration of Anna Perenna - Goddess of the Eternal Year in Roman tradition. So that the circle could be completed, offerings were made to the Spirit of the Year.
According to Roman mythology, Anna Perenna was Dido's sister who, after Dido's suicide, was driven from Carthage to Latium. She was carried off by Numicus (the God of a stream) having incurred the wrath of Aeneas's wife in Latium. Aeneas's servants follwed her tracks and discovered that she had metamorphosed into a water nymph.
In Roman tradition, 15th March is also the Ides of March, as well as a Festival to Jupiter. Shakespeare had a soothsayer warn Julius Caesar 'Beware the Ides of March' and reputedly he was indeed assassinated on 15th March 44 BC.
The illustration to the right (from the Wikipedia page for the Ides of March) shows a commemorative coin from Autumn of 42 BC, issued by Caesar's assassin Brutus, depicting the abbreviation EID MAR (Ides of March) under a Cap of Freedom between two daggers.