At the top of the page is a large KL (for Kalendae), followed by the name of the month, October (f. 310r). The vertical green stripe with red borders in the outer margin is purely decorative.
The numbers in the first column and the letters in the second are a guide to the date of Easter (which varies from year to year). The numbers and letters in columns three and four show days of the months. In the fifth column are the names of saints in capital letters commemorated on particular days. The surrounding smaller names are usually Bohemians (all of whom were dead), including historical figures and important local ones, some of whom were probably benefactors to the monastery where the Codex Gigas was made, and the names of monks and nuns.
The Calendar in the Codex Gigas is unusual for its date in that every day has entries. It is also unusual for including the names of local figures. The system of numberng the days is peculiar for modern readers, but it is easy to understand here as the first line corresponds to the first day of the month, and the last line the last day.