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.)