pie-2
Explanation
This comic is a linguistics joke built on the concept of Proto-Indo-European (PIE), the reconstructed ancestor language from which most European and many Asian languages descend. Two scholars in historical-looking garb announce that they are creating "PIE Day" -- a holiday for linguists, analogous to Pi Day (March 14, celebrated by mathematicians).
One scholar admits they don't yet know the traditions of Proto-Indo-European Day, but they can "reconstruct them from later holidays" -- a direct reference to the comparative method used in historical linguistics, where features of the proto-language are reconstructed by comparing descendant languages.
The caption delivers the full punchline: "The only rule of Proto-Indo-European Day is that each food, drink, or song must be the ancestor of at least two found in different modern holidays." This mirrors the actual methodology of comparative linguistics, where a feature is only reconstructed for the proto-language if cognates appear in at least two separate daughter language branches. The joke is delightfully nerdy, applying rigorous linguistic reconstruction methodology to party planning. Published on March 29, it also appears near the actual Pi Day (March 14), adding a calendrical layer to the wordplay.