What term describes the first day of every month in ancient Rome?

The term that describes the first day of every month in ancient Rome is "Kalends." In the Roman calendar, the Kalends always referred to the first day of each month and was significant in Roman religious and civil life. It marked the beginning of the month and was used for both financial and agricultural purposes.

The other terms have specific meanings as well: the Ides typically refers to the middle of the month (the 13th or 15th, depending on the month) and were often associated with the full moon. The Nonae, which means the ninth day before the Ides, usually occurs on the 5th day of the month (or the 7th in March, May, July, and October). Decima is not a recognized term in Roman calendrical contexts and does not pertain to days of the month. Thus, Kalends is the correct term for the first day of every month, reflecting its importance in the organization and culture of ancient Rome.