The phrase "when the day of Pentecost was fully come" in Acts 2:1 does not support the observance of Pentecost on a Monday. The word "Pentecost" means the 50th day, not the 51st. The Greek word translated as "fully" in this verse is sumpleerousthai, an infinitive form of sumpleero, meaning to fulfill or fill full. This word, combined with the preposition en and the definite article, forms a present tense articular infinitive, indicating that the action of fulfilling was ongoing at the time. Therefore, the phrase should be understood as "during the accomplishing of the day, the 50th day," which was a Sunday. This interpretation aligns with translations such as "While the Day of Pentecost was running its course" and "During the course of the Day of Pentecost," confirming that the 50th day was being accomplished on a Sunday, not completed on a Monday.