Most obvious answer is four more Gods. Quite a few of the months were named after Gods or Goddesses.
After all...January = Janus, March = Mars, May = Maia (maybe), June = Juno
Actually, the original names for July and August were Quintilis and Sextilis (5th and 6th.... the original Roman calendar started from March). The Roman calendar predates Caesar by several centuries, he just renamed those two months (to Iulius and Augustus - July and August), added and removed days from months, and removed a 13th "Intercalendar" month.