Advance Acclimatization
Why is Shavuos just one day? Note the explanation given in Torah Or: “The reason why Shavuos is but a single day—unlike Pesach and Sukkos, which span seven days—is because we then receive the Torah. Torah stems from the realm of Supernal Kesser (‘crown’), which is also the level of Yechidah (‘only one’—the highest tier of the soul’s sefirot), a level that is united with G-d. Kesser stands beyond division into seven midos (the supernal attributes that are far ‘lower’ than Kesser), which are called ‘the seven supernal days.’”
Second day of Shavuos: To summarize, the spiritual source of the weekly cycle is the seven sefirot, and these levels are revealed on the seven days of Pesach and Sukkos. Shavuos, however, marks the revelation of Torah, whose source is the transcendent level of Kesser—and like Kesser, the festival is but a single, supreme day.
Note that even Rosh Hashanah is not one day—but two days. This is the case even within the Land of Israel. True, we consider the two days of Rosh Hashanah “as one long day,” nevertheless, we recite a separate Kiddush at the beginning of each day, clearly defining it as a two-day festival. Conversely, the festival of Shavuos is unique in being just one day. The reason for this is because Shavuos reflects a superior level that is indivisible.
