E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The sixth month (beginning from March) was once called sextlis, but was changed to Augustus in compliment to Augustus Cæsar of Rome, whose lucky month it was, in which occurred many of his most fortunate events.
The preceding month (July), originally called Quintlis, had already been changed to Julius in honour of Julius Cæsar.