I have lived in Israel for most of my life. Many mitzvot are only relevant in the land of Israel, but there is one question that only crossed my mind once I left my country. I had not contended with the issue of what to do when traveling abroad for a holiday. I knew that there were differing opinions but on the rare occasion when I did travel abroad, I followed a psak, halakhic ruling, to observe only one day of the holiday, while being careful not to do any melakha, prohibited activities, publicly in a Jewish community on the second day of the holiday.
However, this issue came to an abrupt head when I moved to the U.S. for a period of a few years to study at Yeshivat Maharat. During my first Sukkot in the U.S., I observed one day but felt an unsettling disquiet within. I was eventually able to put a name to it – I felt lacking in my halakhic integrity. As a future Maharat, it was time for me to do my own research and find out what was really going on behind the scenes of the halakha.
I had heard of a ruling requiring all Jews to observe one day while in Israel and two days when outside of Israel. This made sense to me as it matched the original customs observed within and without the land of Israel and seemed the best way to commemorate those customs.
The lunar month is either 29 or 30 days long. During the time of the Sanhedrin (supreme rabbinic court), Rosh Chodesh, the first day of the new month, was determined by eyewitnesses who actually saw the new moon. They would report to the Sanhedrin, which would then determine the date for Rosh Chodesh, and send out messengers to notify all the Jews living in Israel and in the diaspora of the appropriate date. These communities would then celebrate Sukkot and Pesach on the fifteenth of Tishrei and Nissan and subsequently count 49 days to Shavuot on the sixth of Sivan. The messengers always had enough time to reach the communities in Israel before the fifteenth of each month. However, the messengers would reach communities outside of Israel after the fifteenth of the month, which left them with a doubt as to the correct day to celebrate each holiday. They therefore observed two days of chag, just in case.
Once the Jewish calendar was set (sometime between 400 and 500 CE), our sages instructed these same communities outside of Israel to continue observing two days of the holiday. This was so that they would not forget customs unique to observing two days of the holiday, lest we lose track of the established Jewish calendar or a foreign government not allow us to observe the holidays on the proper date.
One Day in Israel
Visitors to Israel have myriad options. Many halakhic decisors opine that one should observe two days, based on Mishna Pesachim 4:1. This Mishna says that a visitor must observe the stringencies of the land from which she came as well as those of the land which she is visiting. According to this logic, visitors to Israel must observe two days in Israel because that is the custom of the communities from which they came. However, the Chacham Tzvi’s brilliant read of the Mishna in Pesachim leads him to a different conclusion.
The Chacham Tzvi explains that this rule applies only when comparing “apples to apples.” In other words, when the circumstances are exactly the same in both places but the custom itself differs. However, the case of one vs. two days of the holiday is not simply a personal custom observed differently in Israel and in the diaspora; rather, because communities in Israel never had any doubt as to the correct day of the holiday, it was never relevant for them to observe two days. The custom of observing two days of the holiday is geographically linked only to the diaspora and therefore the Mishna’s imperative to keep both the local custom and your home community’s custom does not apply when visitors come to Israel for a holiday. The Chacham Tzvi posits that everyone should observe one day while in Israel. He even suggests that one who does observe two days in Israel risks violating bal tosif, the prohibition against adding commandments to the Torah.
Intuitively it seemed that this same logic of the Chacham Tzvi would be applied in the other direction. I was growing more and more sure that the correct ruling would be for me to observe two days outside of Israel – no simple task for an Israeli. But again I noticed an unsettled feeling as I continued to research the issue. It took some introspection and hard thinking before it came to me in a flash. Of course! It was difficult for me to give a ruling for myself as I would be directly affected by the decision. I needed to continue my research as if someone else had asked me this halakhic question.
Amazingly, this simple realization eased my tension immediately and I returned to my halakhic journey with renewed enthusiasm.
 Beitza 4b: see Rashi who explains why two days were observed in the Diaspora as it was too far for the messengers to get there before the fifteenth of the month
 This includes issues such as saying shehchiyanu, preparing from one day to the next, different Torah readings, when to say yizkor and others. In some communities burial may take place on the second day.
 Shulchan Aruch HaRav 496:11, Mishna Brura 496:13, Iggerot Moshe, Orach Chaim 4:101, and others
 Rav Tzvi Hirsh Ashkenazi (1660–1718), Responsa 167