Rosh HaShanah, Yom Kippur, and Sukkos are all in Tishrei, which has 30 days.

Pesach is in Nissan, which has 30 days.

Shavuos is in Sivan, which has 30 days.

What is the significance of this fact, that every month that has a Biblical holiday in it is a 30-day month? (Although month lengths weren't predetermined in the times of Tanach, I thought it was an interesting observation that later when the set calendar was made, it was arranged to be this way.)

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    Probably because they always come after months with 29 days. if they varied then Yom Tov sheni would become Yom tov rishon. That's sufficient for me. The relevant Talmudic passage for these rules is RH 19b – Double AA Jul 23 '18 at 16:42
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    @doniel I don't understand where I said Nissan varies. If 1 of Adar [II] or Elul would have 30 the safek day of Yom Tov would come before the Vadai day, unlike the other set of holidays, which would be weird/confusing. Sivan is two months after Nissan so it's very likely the same length. So it's no surprise that all three months are the same length. It's expected! – Double AA Jul 23 '18 at 17:04
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    @DoubleAA This is a very good answer! Why don't you put it in the answer section? Are answers which are definately correct, but not sourced by anyone else, not valid by MY rules? How about if you wrote it on "yoursite", and then answered here " Rabbi DoubleAA says HERE (linked) that..." ? Would it suddenly become valid? – RibbisRabbiAndMore Jul 23 '18 at 17:08
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    Interesting question. But there are two slight "problems" - 1) As you stated, the Biblical holidays were not based on any notion of a specific month length. So, my own thinking is that it is coincidental (OK, Hashem planned that Hillel II should make the calendar alternate the way that he did.) and 2) Shabbat is also a Biblical holiday. It doesn't always occur in a 30-day month. – DanF Jul 23 '18 at 19:22
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    Nothing, 1. in times of Kiddush Hachodesh, it was totally arbitrary and in the hands of the Beis Din. 2. when the fixed calendar was introduced, the main idea was to count full and lack months alternatively - full/lack/full/lack, starting with Elul. Those 3 festivals fall on 1, 7, 9 months and are automatically full. – Al Berko Jul 23 '18 at 19:31

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