This was the start of the Floralia, sort of a May Day celebration, in honor of the goddess of flowers, with public games, mimes, and other theatrical performances.
Ludi Floralia
It was also the birthday of the Emperor Otho in A.D. 32. Marcus Salvius Otho was one of the four emperors in the year A.D. 69. Otho was princeps from 15 January-15 March.
Plutarch's Life of Otho