"And you shall celebrate it as a festival for the L–rd": This tells me only of the first day of the festival as requiring a chagigah (offering). Whence do I derive (the same for) the last day? It is, therefore, written (Ibid. 13:6) "and on the seventh day, a festival (chag) to the L–rd." This tells me only of the first and the last days as requiring a chagigah. Whence do I derive the same for the intermediate days of the festival (chol hamoed )? It follows inductively, viz.: Since the first and the last days are called mikra kodesh ("a calling of holiness") and chol hamoed is called mikra kodesh, then if you have learned about the former that they require a chagigah, then the latter, too, require a chagigah.