Y"T sheni and sfeika d'yoma

Two points that came up in comments that re worth their own post in case you missed them -- first a short Netziv: The Shi'iltos interprets the pasuk "ushemartem mitzvosai..." (Vayikra 22:31) which appears in the context of the moadim as a command to make a seyag laTorah. Netziv writes that this pasuk is the basis for the institution of the second day of Y"T for those living outside Eretz Yisrael. Even though it is commonly thought that Y"T sheni was instituted because of sfeika d'yoma, i.e. the news of when Rosh Chodesh occurred was slow to reach outside Eretz Yisrael and therefore there existed a doubt as to which day of Y"T is the correct date, in reality this is not sufficient reason for the takanah. In the overwhelming majority of the time the months of Adar and Elul have only 29 days. The principle of rov dictates that there is no need for concern for the chance occurance that things might be different. Proof that we rely on this fact: we celebrate one day of Yom Kippur and not two despite the same "safeik" as would apply to any Yom Tov existing; we count sefiras ha'omer from the second night of Pesach as if we were certain that this is the correct night to start counting. Only with respect to the moadim where there exists this extra command to make a seyag that Chazal instituted an extra day.