**Note -- sorry about the lack of hyperlinks. I wrote this in Word and C/P it here. I tried to make the included references into links but got bored.
A while ago, I was considering whether to go to Israel for an upcoming holiday, one that in the US would be celebrated for 2 days (https://en.wikipedia.org/wiki/Yom_tov_sheni_shel_galuyot), but only for 1 in Israel (https://www.torahweb.org/torah/special/2003/rsch_ytsheini.html). The practice of the second day developed outside of Israel because of a concern over the exact date of the new month (http://www.chabad.org/library/article_cdo/aid/527614/jewish/Why-Do-We-Still-Celebrate-Holidays-for-Two-Days-in-the-Diaspora.htm) declaration (which would in turn affect the certainty of the date of a holiday later in the month). In Israel, where there was no uncertainty, no extra day was needed. This practice was so established (http://halachipedia.com/index.php?title=Second_Day_of_Yom_Tov) that when Hillel II fixed the calendar scientifically (http://www.aish.com/atr/Two_Days_of_Yom_Tov.html), the 2 day practice was institutionalized. But what is still argued is whether someone visiting Israel has to celebrate two days, as his minhag (http://traditionarchive.org/news/originals/Volume%2013/No.%203/The%20Second%20Day%20of%20Yom%20Tov.pdf) (sort of “tradition”, sort of “accepted practice) would dictate, or one day because “when in Rome” (http://www.yeshiva.co/ask/?cat=257). The underlying question seems to be whether the tradition is incumbent on the person or the place and which, in any situation is dominant if the two practices are different.
Minhag avoteichem b’yadeichem (being careful about keeping family traditions) -- http://www.torahmusings.com/2015/02/binding-minhag/
Minhag hamakom (customs that are based in adhering to a local practice) - http://thejewishreview.org/articles/?id=212
Certain practices are clearly subject to the location – if one goes to Jerusalem, even as a visitor, over Purim, one celebrates Purim on Shushan Purim, a day after when he would celebrate it were he in an unwalled city. His practice is dictated by where he is. Similarly, a Kohen who visits Israel from the US says the Priestly blessing daily as is the Israeli practice. Regardless of the law which applies to him outside of Israel, a change in location affects how he behaves. The law seems to be on the place, not the person.
I also wanted to donate plasma over chol hamo’ed. I spoke with my rabbi and he said that the understanding of the law dictated that something quasi-medical but not definitively life threatening that could be rescheduled should be. I pointed out that in Israel, the Magen David Adom actually encourages people to donate blood over chol hamo’ed and I’m sure that they do so with the knowledge of the rabbinate. He guessed that the difference is that in Israel, and reduction in donations (because of the size of the country and the constant concern of attack) would immediately and certainly become a life threatening set of conditions. I wondered then, if I were to travel there, would the local understanding of law and application of it apply to me as well – is the issue the way my authority rules or the way a law develops in relationship to the geography of the adherents.
This question also applies then to the question of kitniyot, legumes, on Passover. Ashkenazic Jews have developed a minhag/tradition/practice with the power of law not to eat them. One reason (http://halachipedia.com/index.php?title=Kitniyot) has to do with the confusion over grains when similar objects are stored with each other. Because one could not ensure that a given grain-type product was free of the 5 grains which must be guarded and controlled on Passover, the rule was established to eschew all similar looking products (http://www.torahmusings.com/2016/12/allowing-kitniyot/). But in communities where the offending legumes were not mixed in, or were checked, the practice never took hold (http://en.yhb.org.il/2011/04/08/kitniyot-on-pesach/). There was never any uncertainty about the unadulterated status of the legumes there so there was never any need to prohibit them (http://blogs.timesofisrael.com/why-sephardim-eat-kitniyot-but-ashkenazim-dont/). This is the case with many communities within the Sephardic tradition.
So what if I were to visit some place which historically never established this minhag. Is the law on me or the location? I am not asking about my practice if I eat at someone’s house in Teaneck and the person is Sephardic and eats kitniyot but about being somewhere where there was never the worry which demanded that the tradition even exist. Shouldn’t I be able to eat legumes because the concern over their status – the exact reason why I am forbidden, is completely alien and assuredly not the case! If the law is on me specifically then it needs no reason: I can’t eat legumes simply because of an accident of parentage. But if other practices as listed above switch based on where I am, regardless of my birth, why shouldn’t this.
And then the question of 1 or 2 days of the holiday returns. If there is ample precedent for abiding by the rules of the location, then why would there be any reason for me to celebrate 2 days in Israel? Their understanding and application of the law didn’t have the development of the 2nd day in the same way that their location and conditions provide for a different day for Purim, and an allowance to duchen daily, or give blood on chol hamo’ed. Why am I subject to their practices in certain cases and not in the case of the second day of the holiday, unless one wants to say that the law is on the person, not on the place.
Maybe this all depends on the notion of “law” vs. “tradition” vs. "minhag" and maybe the gradations between and within each are too fine (and they are certainly beyond my understanding), but it seems almost arbitrary to require that certain practices stay with the individual no matter his location while others are flexible if he changes and eliminates the condition of uncertainty which inspired the distinct practice.