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]".[5]

[1] Bamidbar 28:14

[2] Shemos 12:2

[3] Rosh Hashanah 20b;[3] Menachot 29a

[33] This is a test footnote of the second level; TODO REMOVE!

[5] Dvarim 16