(5) In the present era, when the Sanhedrin no longer exists, and the court of Eretz Yisrael 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 Eretz Yisrael 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.

(21) Our celebration of every holiday for two days in the diaspora is merely a custom. For the second day of the holiday is a Rabbinic institution, innovated during the exile. The inhabitants of Eretz Yisrael observe a holiday for two days only on Rosh HaShanah.
In Hilchot Kiddush HaChodesh in this volume the fundamental principles pertaining to this custom and the rationale why Rosh HaShanah is universally observed for two days will be explained.

(5) On each and every one of these eight days, the entire Hallel is recited. Before [its recitation], one should recite the blessing "...who has sanctified us with His commandments and commanded us to complete the Hallel." This applies whether the recitation is individual or communal.
Even though the reading of the Hallel is a mitzvah ordained by the Sages, one may recite the blessing [stating] "who sanctified us with His commandments and commanded us," as one recites a blessing for the reading of the Megillah and for the erection of an eruv. A blessing should be recited for every definite obligation established by our Sages.
In contrast, if an obligation was established by the Sages because of a doubt - e.g., tithing d'mai, - a blessing is not recited. [This principle invites a question:] Why is a blessing recited over the second day of a festival; its observance was ordained only because of doubt? [This was ordained] lest it be treated with disdain.

(16) [A blessing is not recited over] all practices that are customs. [This applies] even to a custom established by the prophets - for example, taking the willow branches on the seventh day of Sukkot. Needless to say, a blessing is not recited over customs established by the Sages - e.g., reading Hallel on Rosh Chodesh and on the intermediate days of Pesach.
Similarly whenever there is a question whether a practice requires a blessing or not, it should be performed without reciting a blessing.
A person should always take care not to recite blessings that are not necessary, and should recite many blessings that are required. Thus, David declared [Psalms 145:2]: "I will bless you each day."

(26) Similarly, it includes the customs and ordinances that were ordained or practiced in each generation according to [the judgment of] the governing court of that generation. It is forbidden to deviate from [these decisions], as [implied by Deuteronomy 17:11]: "Do not deviate from the instructions that they will give you, left or right."

(2) Any person who does not carry out their directives transgresses a negative commandment, as Ibid. continues: "Do not deviate from any of the statements they relate to you, neither right nor left."
Lashes are not given for the violation of this prohibition, because it also serves as a warning for a transgression punishable by execution by the court. For when a sage rebels against the words of the court, he should be executed by strangulation, as the following verse states: "A person who will act deliberately...."
We are obligated to heed their words whether they:
a) learned them from the Oral Tradition, i.e., the Oral Law,
b) derived them on the basis of their own knowledge through one of the attributes of Biblical exegesis and it appeared to them that this is the correct interpretation of the matter,
c) instituted the matter as a safeguard for the Torah, as was necessary at a specific time. These are the decrees, edicts, and customs instituted by the Sages.
It is a positive commandment to heed the court with regard to each of these three matters. A person who transgresses any of these types of directives transgresses a negative commandment. This is derived from the continuation of the above verse in the following manner: "According to the laws which they shall instruct you" - this refers to the edicts, decrees, and customs which they instruct people at large to observe to strengthen the faith and perfect the world. "According to the judgment which they relate" - this refers to the matters which they derive through logical analysis employing one of the methods of Biblical exegesis. "From all things that they will tell you" - This refers to the tradition which they received one person from another.

(7) In places where the festivals are celebrated for two days, Hallel is recited on 21 days: On the nine days of Sukkot, the eight days of Chanukah, the [first] two days of Pesach, and the two days of Shavuot.
[In contrast,] the recitation of Hallel on Rosh Chodesh is a custom and not a mitzvah. It is observed [only] communally. [To emphasize that it is a custom,] passages are skipped when it is read. A blessing should not be recited over [this reading], since a blessing is not recited over a custom.
A person [praying] alone should not recite [the Hallel] at all [on Rosh Chodesh]. If, however, he began its recitation, he should complete it, skipping the passages the community would skip as he reads it.
Similarly, on the other days of Pesach, [the Hallel] is read while skipping passages.

(12) There are places where the messengers [sent out for] Nisan would arrive [in time for the celebration of Pesach], but the messengers [sent out for] Tishrei would not arrive [in time for the celebration of Sukkot]. According to [the letter of] the law, it would have been appropriate for them to observe Pesach for one day, since the messengers reached them and informed them when Rosh Chodesh had been established, and for them to observe Sukkot for two days, since the messengers had not reached them. Nevertheless, so that there would be no difference between the festivals, the Sages instituted the ruling that two days were celebrated [for all holidays] in all places that were not reached by the messengers for Tishrei. [This includes] even the festival of Shavuot.

(1) You have asked: How should people from outside Eretz Yisrael (hutz la-aretz) who temporarily visit there practice during the three festivals, as residents of Eretz Yisrael or as residents of hutz la-aretz?

(2) Response: In my humble opinion, on matters of festivals they must practice like a resident of Eretz Yisrael. This is not in the category of “the stringencies of the locale one left” [which must be observed]. This goes without saying with regard to the blessings, prayers, and Torah reading, which are not essentially stringencies, for if one practices stringency by reciting the blessings and prayers of the festival on a day that is not a festival, he has transgressed. However, it is even permissible for them to perform melakhot (labors forbidden on Shabbat and festivals), for if all of the people from the locale they left would move here permanently, it is obvious that they would not be allowed to observe more than one day, as this would violate “you shall not add on” (bal tosif). After all, one who sleeps in the sukkah on the eighth day incurs lashes. Similarly, on Pesah and Shavu’ot, one who observes one festival day more than is commanded violates bal tosif. [The Sages] did not say that “we impose upon him the stringencies of the locale he left” except with regard to a stringency that the people from the community one left may practice in a locale that practices leniently even if they would establish permanent residence in the latter locale. However, when it comes to something that the people from the stringent locale would be forbidden to practice if they were to establish residence in the lenient locale, [the Sages] did not make their ruling about such a case. And although “they sent [a message] from there: be careful about the practices of your fathers, lest there be decrees of persecution that will be ruinous” (Beitza 4b), this ruin itself is only relevant when they are in their place, in hutz la-aretz. However, while they are in Eretz Yisrael, it does not apply. And since in Eretz Yisrael it is forbidden to add a day to the mitzva, and the people of Eretz Yisrael do not have the capability of adding a day to what the Torah states and thus be stringent, even those who come from hutz la-aretz may not practice the second festive day of Diaspora communities while they are in Eretz Yisrael, even if they are there only temporarily, since the determining factor is the locale, and it is not in the category of “stringencies of the locale he left.”

(3) I have written my humble opinion. Tzvi Ashkenazi

(1) Yom Tov Sheni / A Diaspora Resident on a Visit or Extended Stay to Eretz Yisrael

(2) There is an opinion that one visiting Eretz Yisrael from the Diaspora is considered to be a resident of Eretz Yisrael for the duration of his stay (Ḥakham Tzvi §167). However, most poskim rule that since he lives abroad, he is considered a Diaspora resident even while visiting Eretz Yisrael. This is the opinion followed in practice (Birkei Yosef 496:7; MB ad loc. 13).

(3) However, if an additional uncertainty arises – for example, if he expects to remain in Eretz Yisrael for an extended period of time, plans to make aliya, or has children living in Eretz Yisrael – then we combine this uncertainty with two other factors. First is the opinion above that everyone who is in Eretz Yisrael for Yom Tov should keep one day. Second is that, in recent times, there are increased chances that a Jew visiting Eretz Yisrael will decide to relocate there permanently. Combining these three factors, we instruct such a person to follow the practice of those who live in Eretz Yisrael.

(4) Therefore, one who comes to Eretz Yisrael for an entire academic year is considered a resident of Eretz Yisrael, even if he has definite plans to return to the Diaspora, his parents live there, and he goes to visit them mid-year. His extended stay in Eretz Yisrael renders him a resident of Eretz Yisrael for the duration of his stay. Additionally, there is always the chance that he will decide to make aliya, given that there is a Torah commandment to live in Eretz Yisrael.

(5) However, if one comes for a shorter visit, even up to half a year, and plans to go back to the Diaspora, he is considered a Diaspora resident. If he visits repeatedly, then once his visits cumulatively add up to a year, he is something of a local, and there is a certain chance that he will make aliya. Accordingly, when he is in Eretz Yisrael for Yom Tov, he may celebrate the way that residents of Eretz Yisrael do and keep one day.

(6) If one is visiting Eretz Yisrael, even for a short time, but is planning to make aliya as soon as he can, then even if he will not be able to bring his plans to fruition for a number of years, his status while visiting is that of a resident of Eretz Yisrael.

(7) If one visiting has children or parents already living in Eretz Yisrael, he is considered a resident of Eretz Yisrael while visiting. Even if he does not intend to make aliya, nevertheless he has deep family ties to Eretz Yisrael, so there is always a chance that he will make aliya.

(8) If one buys an apartment in Eretz Yisrael and lives there during his visits, he is considered a resident of Eretz Yisrael while there.