Positive commandment · Mitzvat Aseh #153 of 613
Sanctifying the new month
לקדש חודשים ולחשב שנים וחודשים בבית דין בלבד
The verse · Shemot 12:2
הַחֹדֶשׁ הַזֶּה לָכֶם רֹאשׁ חֳדָשִׁים רִאשׁוֹן הוּא לָכֶם לְחׇדְשֵׁי הַשָּׁנָה
“This month shall be to you the beginning of months: it shall be the first month of the year to you.”
The Rambam’s words
To sanctify the months and to calculate the years and months. [This mitzvah is incumbent on] the court alone, as [Exodus 12:2] states: "This month will be for you the first of the months."
לקדש חודשים ולחשב שנים וחודשים בבית דין בלבד, שנאמר: "החודש הזה לכם, ראש חודשים" (שמות יב,ב).
Why · Sefer HaChinuch #4
Sanctifying the new month
“It is from the roots of this commandment [that it is] so that Israel shall make G-d’s holidays in their (proper) times, as G-d commanded that we make Pesach in the time that the produce is in [bloom], as it is written (Deuteronomy 16:1), “Observe the month of Spring, and you shall offer the Pesach sacrifice.””
Who & when
“And [it] is practiced in every place and at all times when we have ordained sages, according to the aforementioned conditions.”
Where in the Mishneh Torah
Hilchot Sanctification of the New Month הלכות קידוש החודש