Shemini Atzeret is a holiday dedicated traditionally to a core Jewish principle–so core it is at the center* of daily morning prayers (although not in a Reform prayerbook, unfortunately): that our actions affect the climate, including whether we get the rainfall we need. It’s not a crazy liberal-lefty idea; it’s an ancient religious one, more relevant toady than ever.
An article in JTA, “Why Shemini Atzeret is the pinnacle of the High Holidays season“, summarizes why I find the “closing” holiday of this season so meaningful. It is sad, in a way to me, that it starts with the phrase, “You might not know it…” Indeed, the holiday seems almost unknown to many Jews who do not engage in synagogue life or who do so through a Reform framework.** Knowledge of the meaning of the holiday is not helped by the fact that Simchat Torah–which really just originated as a way to celebrate second-day Shemini Atzeret in the Diaspora–has taken over the original holiday’s purpose, especially but not only in Reform Jewish communities in the US. Yet it is actually a holiday with a very important, central message, to be celebrated and taken seriously in its own right.
* See the section at that link that is from Deuteronomy 11.
** Nothing against Reform, per se. They get many things right, including not having two days of holidays (other than Rosh HaShannah).