The Julian calendar
The calendar of 365 days, with a leap day every fourth year, that Julius Caesar set up from 45 BCE. England kept it until 1752; its dates ran ten days behind the reformed Gregorian calendar in the 1600s and eleven days behind after 1700.
How the Romans fixed the year
Censorinus, On the Birthday 20–22 (238 CE), in the edition of Friedrich Hultsch (Leipzig, 1867) · 238 CE
quapropter nunc cum in septem mensibus dies singuli et triceni sint, quattuor tamen illi ita primitus instituti eo dinoscuntur quod nonas habent septimanas, ceteri tres quintanas. praeterea pro quadrante diei, qui annum verum suppleturus videbatur, instituit ut peracto quadrienni circuitu dies unus, ubi mensis quondam solebat, post terminalia intercalaretur, quod nunc bissextum vocatur.
So today seven months have 31 days. But the four that had 31 days from the start can still be told apart. Their Nones fall on the seventh day, while the other three have them on the fifth. Then there was the quarter of a day that seemed to be needed to complete the true year. For this he ruled that, at the end of each four-year cycle, one day should be inserted after the Terminalia, where the extra month used to go. This day is now called the bissextum.
ex hoc anno ita a Iulio Caesare ordinato ceteri ad nostram memoriam Iuliani appellantur, eique consurgunt ex quarto Caesaris consulatu. qui etiam si optimo , non soli tamen ad annum naturae aptati sunt: nam et priores anni, etiam si qui decemmenstres fuerunt, nec Romae modo vel per Italiam, sed et aput gentes omnes, quantum poterat, idem fuerunt correcti.
The years since then, down to our own time, are called Julian after this year that Julius Caesar set in order. They are counted from Caesar's fourth consulship. Even if these are the best [words are lost here], they are not the only years fitted to the year of nature. The earlier years too, even those of ten months, were corrected in the same way as far as possible, not only at Rome or in Italy but among all peoples.
eorum vero annorum quibus Iulianis nomen est ducentesimus octogensimus tertius, sed ex die Kal. Ianuariarum, unde Iulius Caesar anni a se constituti fecit principium;
Of the years called Julian, it is the 283rd, counting from January 1, the day Julius Caesar made the start of the year he set up.
Where the months got their names
Macrobius, Saturnalia 1.12–14 (early fifth century CE), in the edition of Franz Eyssenhardt (Leipzig, 1893) · early fifth century CE; the dialogue is set in the 380s
Iulius ergo Caesar decem dies obseruationi ueteri super adiecit, ut annum trecenti sexaginta quinque dies, quibus sol lustrat zodiacum, efficerent, et ne quadrans deesset, statuit, ut quarto quoque anno sacerdotes, qui curabant mensibus ac diebus, unum intercalarent diem, eo scilicet mense ac loco, quo etiam apud ueteres mensis intercalabatur, id est ante quinque ultimos Februarii mensis dies, idque bissextum censuit nominandum.
So Julius Caesar added ten days to the old count, to make a year of 365 days, the time the sun takes to pass through the zodiac. And so that the quarter day would not be lost, he ruled that every fourth year the priests in charge of the months and days should insert one day. It went into the same month and place where the ancients had inserted the extra month, before the last five days of February. He decreed that it should be called the bissextum.