Why Two Days of Holiday Outside Israel?
In Biblical times, the dates of Jewish holidays were based on the sighting of the moon. Rosh Chodesh, the first of the month, was set by the court in Jerusalem, when witnesses testified that the new moon appeared in the sky, marking the beginning of the new month. By the time the fifteenth of the month (the dates of the Passover and Sukkot festivals) would arrive everyone in Israel knew when Rosh Chodesh and by extension, when the date of the holiday was. Yet, the communities outside of Israel were left in doubt as to the start of the holiday, as the news of the precise date of Rosh Chodesh would not reach them in time. Therefore, the communities outside of Israel would celebrate two days, taking into account the two days that Rosh Chodesh could occur (depending on whether the previous month had 29 or 30 days), while the Jews in Israel would celebrate only one day. Even today, when the calendar is set and we know the precise day of the holidays, Jews outside of Israel celebrate two days of the holiday to perpetuate the custom and practice of their ancestors. As Maimonides explains:
Wherever these messengers would arrive [before the celebration of the festivals], the holidays would be observed for [only] one day, as prescribed by the Torah. In the distant places, which the messengers would not reach [before the celebration of the festivals], the holidays would be observed for two days because of the doubt [involved]. For they would not know the day on which the High Court established the new month.
…In the present era, when the Sanhedrin no longer exists, and the court of the Land of Israel establishes [the months] according to the [fixed] calendar,according to law, it would be appropriate for [Jews] throughout the world to celebrate the holidays for one day alone. For [the inhabitants of] the distant regions of the diaspora and the inhabitants of Israel rely on the same [fixed] calendar and establish [the festivals] accordingly. Nevertheless, the Sages ordained [that the inhabitants of the diaspora] retain the custom of their ancestors. (Laws of Sanctification of the New Moon, Chapters 3 and 5)
The mystics explain that in addition to the historical, technical, reason, there is also a deeper spiritual reason. The holy land of Israel is far more intune to the spiritual holiness of the holiday. Therefore, for the Jewish people in Israel, one day is enough to connect to and internalize the spiritual energy of the day. In contrast, outside of Israel, there are additional layers of concealment of the Divine energy and light, therefore one day is not sufficient and more time and effort is required to achieve that which can be achieved in Israel in one day.
While the mystical perspective emphasizes the disadvantage of the diaspora, the Chassidic perspective, taught by the Rebbe, transforms that disadvantage into an advantage. While outside of Israel, we are more distant from holiness, we are farther from the holiness manifest in Israel. Yet that distance creates a deeper longing and yearning for holiness. Then, when the holiday finally arrives, and we sense a measure of closeness to G-d, our joy is far greater than the joy of those in Israel who did not feel the pain of distance as potently. The increased, intense joy, born from the yearning due to the distance, is expressed in the additional day of holiness and celebration outside of Israel.
Adapted from the teachings of the Rebbe, Toras Menachem vol. 74 p. 119