The Kalends is the first day of the month in the Roman calendar. When Roma Antiqua followed a lunar calendar, the Kalends was the day of the New Moon. However, when the calendar of Roma Antiqua underwent numerous alterations over the years, the Kalends became the 1st day of the month instead; derailing it from coinciding with the New Moon.
History of the Ritual
On this day in Roma Antiqua, the Rex Sacrorum was accompanied by a secondary Pontifex in making a sacrifice in the Curia Calabra on the Capitoline Hill. This sacrifice was made to Iuno and also honored Ianus; the prevailing idea being that Ianus, as the God of Good Beginnings, assisted Juno in giving "birth" to the new month.
In the course of this sacrifice, the name of Juno Covella was invoked as the date of the Nones was announced. After the Rex Sacrorum fulfilled his duties in performing this sacrifice, his wife the Regina Sacrorum would perform a sacrifice to Juno in the Regia in the Forum.
The modern ritual for Cultores may be found here: Kalends ritual (Nova Roma)