This study examined estimation and accuracy of normative perceptions for students during one celebratory drinking occasion. Students who drank on St. Patrick's Day overestimated the percentage of others who also drank, whereas the students who did not drink on St. Patrick's Day underestimated the percentage of others who also did not drink. The students' drinking behaviors on St. Patrick's Day did not substantially differentiate their accuracy scores. However, of the students who drank on St. Patrick's Day, those who did not believe the ads showed stronger overestimation of others who drank than did those who believed the ads. (Contains 1 table.)