Lex Acilia de intercalando (The Acilian Law Concerning Intercalation) was a law established in ancient Rome in 191 BC.
Introduced by the consul M. Acilius, this law probably authorized the pontifices to decree an intercalary period in order to prevent seasonal drift in the pre-Julian lunar calendar. While the historian Fulvius claims that this law was the first instance of intercalation in Roman history, the historian Varro cites instances as early as the 5th century BC. It is also alleged that the decemviri were the first to intercalate, a practice they may have learned from the Etruscans.