Actually, it has been reported that, Elder, Armstrong, for a short time, at the very beginning of his conversion kept a Sivan 6 Pentecost as by the Hebrew Calendar observed by the majority of the Jewish community through out the world. His change to an always Monday Pentecost and then to a Sunday Pentecost (as the Karaite Jews observe) in the early 1970s, was based on a question that had arisen about the proper point of where to start the count from. Once, Elder, Armstrong made the decision to change from a Monday to a Sunday Pentecost there remained a certain contingent of loyalists who persisted that, Elder, Armstrong made a mistake and continued to observe a Monday Pentecost, which no Jew adheres to. The why then, as to the many Church of God groups keeping a Sunday Pentecost is to, Elder, Armstrong's change and influence in His teaching on the issue as to at what point one is to start the count from. For a more detailed explanation as to the specifics on this issue go to: http://www.triumphpro.com/pen_hwa.htm Read the article titled "Did Herbert Armstrong Ever Observe Pentecost on Sivan 6?" by William F. Dankenbring.