The Calendar of Harptos

The Calendar of Harptos is the calendar used across most of Faerûn. It was created by the wizard Harptos of Kaalinth, although most people associate the word "Harptos" with the calendar itself. It is split into twelve months, each lasting three tendays (or thirty days). There are an additional five days that fall between months, bringing the total number of days in most years to 365.
The days making up a tenday do not have formal names. If precision is required, the number of the day and the number of the tenday is used, as in, "the fourth day of the first tenday of Flamerule". Days of the month are typically written as the numerical date followed by the month name, for example, "15 Hammer" or "15th Hammer". Informally or poetically this could be spoken or written as "the 15th of Deepwinter".