Law 1

The months of the year are lunar months, as it is said: "[...] the burnt offering of the month in its month" [1] and it is said: "This month shall be for you the head of the months" [2]. This is what the sages said: "The Holy One, blessed be He, showed to Moshe, in a prophetic vision, the image of the Moon, and said to him: '[When] you see [the Moon] like this - sanctify [it].'"[3]

But the years that we reckon are solar years, as it is said: "Keep the month of aviv [spring]". [4]

[1] Bamidbar 28:14

[2] Shemos 12:2

[3] Rosh Hashanah 20b; Menachot 29a

[4] Dvarim 16