R. Nathan says "Observe the month of Aviv"—Observe the month which is closest to Aviv. And which is that? Adar. But we have not heard how many (days) are to be intercalated. From "Observe the month (of Aviv"), we infer that a month (of days, i.e., thirty) are to be intercalated. And this follows, viz.: A month is intercalated and a year is intercalated. Just as a month, (with) one of its units (i.e., days), so, a year, with one of its units (i.e., months). But perhaps, just as (with a) month—one thirtieth of it, so, (with) a year, one thirtieth of it? It is, therefore, written "Observe the month." (A month is intercalated and not one thirtieth of it. But perhaps, just as a year, one twelfth of it, so, a month, one twelfth of it? It is, therefore, written (Numbers 28:17) "And on the fifteenth day of this month"—You intercalate a day for a month, and not one twelfth of it.