August...

After Julius's grandnephew Augustus defeated Marc Antony and Cleopatra, and became emperor of Rome, the Roman Senate decided that he too should have a month named after him. The month Sextillus (sex = six) was chosen for Augustus, and the senate justified its actions in the following resolution:

Whereas the Emperor Augustus Caesar, in the month of Sextillis . . . thrice entered the city in triumph . . . and in the same month Egypt was brought under the authority of the Roman people, and in the same month an end was put to the civil wars; and whereas for these reasons the said month is, and has been, most fortunate to this empire, it is hereby decreed by the senate that the said month shall be called Augustus.

Not only did the Senate name a month after Augustus, but it decided that since Julius's month, July, had 31 days, Augustus's month should equal it: under the Julian calendar, the months alternated evenly between 30 and 31 days (with the exception of February), which made August 30 days long. So, instead of August having a mere 30 days, it was lengthened to 31, preventing anyone from claiming that Emperor Augustus was saddled with an inferior month.

To accommodate this change two other calendrical adjustments were necessary:

--> The extra day needed to inflate the importance of August was taken from February, which originally had 29 days (30 in a leap year), and was now reduced to 28 days (29 in a leap year).

--> Since the months evenly alternated between 30 and 31 days, adding the extra day to August meant that July, August, and September would all have 31 days. So to avoid three long months in a row, the lengths of the last four months were switched around, giving us 30 days in September, April, June, and November.

Among Roman rulers, only Julius and Augustus permanently had months named after them—though this wasn't for lack of trying on the part of later emperors. For a time, May was changed to Claudius and the infamous Nero instituted Neronius for April. But these changes were ephemeral, and only Julius and Augustus have had two-millenia-worth of staying power.

by Borgna Brunner, taken form the below website as of 1 August 2003
http://www.infoplease.com/spot/99aughistory1.html