Observance of the festivals create a problem for an Israeli overseas. The major ones, Pesach (Passover), Sukkot (Tabernacles) and Shavuot (Pentecost) are celebrated for one day in Israel whilst outside of Israel they are celebrated for two days. Historically, outside of Israel there was uncertainty as to which day should be celebrated due to the time it took for word to get out from Israel. As there are certain restrictions on 'work' during the festival, two days were celebrated to be on the safe side. Despite the fact that the calendar has been set for hundreds of years the tradition is maintained.
Logically, in olden times, a Jew travelling outside of Israel would have been subject to the same uncertainty as to when the festival fell and would therefore have kept two days. For a Jew from outside of Israel who happened to be visiting during a festival, it would have been logical to keep one day. Nowadays however, when we keep two days because that's the way it's always been rather than for any practical reason, it's most often the case that Israelis who are outside of Israel for a holiday will still keep one day, whilst non-Israelis who visit over a festival will often keep two.
I find myself in a position therefore of having one day of festivities when all around are celebrating two days. This isn't usually a problem - typically I'll grab some DVDs beforehand and stay at home or head out surreptitiously wearing a cap (which involves a different issue - Ma'arit Ayin - which I may discuss at some other time but not here and now).
We have just celebrated the festival of Shavuot, the first day of which fell on Friday - which means that for those who celebrate two days, the second was today - Shabbat. It's the first time that I've encountered this combination and raises a slightly different set of issues than normal. Whereas usually synagogue is not a problem (on regular days I pray at home in the morning rather than at synagogue), here it presents a problem as it means that my normal Shabbat prayers are different - certain additions are made including a whole addition section - Hallel. The Torah reading is also different as is the kiddush.
I got around the problem in an obvious way - took a "sabbatical" (stayed at home with a book and the monkey). Just to get appropriate approval I checked with an appropriate authority - my father in law (who is not a Rabbi but my it keeps me out of trouble with Mrs G if I'm on the same page as he is) - and he told me without prompting that he'd probably do the same. When it came to kiddush we just did it in two different versions - problem solved.
Looking forward to being home where none of these conflicts exist and I don't have to get upset that we're carrying on another tradition for which there is no longer any rationale.