Avi K: While RH is a Yom Tov, it is also held for two days in EY, not because of Yom Tov Sheini Shel Golius but for a different reason having to do with not accepting the eidus of the two eidim after mincha (and the Korban Tamid Shel Bein HAa’arbaim). Therefore, since in most years Elul is chasser (29 days), people held the 30th day as RH, however, if eidem didn’t come before mincha they held both the 30th day and 31st day as RH (Yoma Arichta). See MB 601 SK 3.
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