Kalendae

Japanese: Kalendae

…In this case, the day is calculated including the base day. The base days are Kalendae (Calendae, the first day of the month, the origin of the word calendar), Nonae (the 7th of March, May, July, and October, and the 5th of other months), and Idus (the 15th of March, May, July, and October, and the 13th of other months).

From [New Year]

…In Judaism, the vernal equinox is the new year in the church year. In pagan Rome, the first day of each month was called the "first day of the month, Kalendae, Calendae" and was given great importance, and from 190 B.C., January 1 was considered the beginning of the year. In the early medieval church, March 25 was the day of the Annunciation and the new year was celebrated. …

*Some of the terminology that mentions "Kalendae" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…その際，第何日は基準の日をも含めて計算する。基準となる日はカレンダエKalendae(Calendae。朔日，カレンダーの語源)，ノナエNonae(3月･5月･7月･10月の7日，その他の月の5日)，イドゥスIdus(3月･5月･7月･10月の15日，その他の月の13日)の3日である。…