I too understood Caesar and Augustus to have taken days from February for their own months; but now checking The Oxford Companion to Classical Literature s.v. Calendar and measure of time, it says Numa Pompilius (successor to Romulus as king) "is said to have added the months of January and February, making a year of twelve months (four of 31 days, seven of 29, February of 28), a total of 355 days", with intercalations to bring it into line with the solar year. This was the calendar that Caesar reformed into essentially the modern one, with the same distribution of days as our own.