A tourist from America, is there any rationale for him keeping only 1 day Yom Tov when in Israel?
The Chacham Tzvi (Shu"t Chacham Tzvi 167) believes that even a tourist should observe only one day of Yom Tov when he is in Israel. The rationale for this is that even when the calendar was set month after month by the Sanhedrin on the basis of witnesses who testified to seeing the new moon, someone who lived far from Jerusalem would keep a second day of Yom Tov only on those Yamim Tovim when he was at home and did not know when the new moon was seen. In a year when he was in Israel he would keep one day. Today, if the entire community outside of israel moved to Israel it would keep only one day for this reason. Most poskim did not accept the opinion of the Chacham Tzvi. Harav Shlomo Zalman Auerbach (Shu"t MInchat Shlomo I 19) explains the reason for this: the decision to keep a second day outside of Israel even at a time when the Jewish calendar is permanently set and no doubt exists anywhere in the world as to the Jewish date, binds the resident of "chutz laaretz" (lands outside of Israel) wherever he may be, even when he is in Israel. One is commanded of course to come on aliya and keep one day. If however fulfilling this mitzva is for some reason impossible, a tourist should keep two days in Israel in line with the majority opinion.