Outside of Israel, Shavuot is a two-day Yom Tov, and both Pesach and Sukkot begin and end with two days of Yom Tov as well. In ancient times, the new month was proclaimed by the Beit Din in Jerusalem. Messages were then sent to the surrounding and outlying communities, telling them when the new month began. Because the more distant communities did not receive the message before the start of the holidays, those living outside Israel observed two days of Yom Tov due to the uncertainty of the correct date. Although today there is a set calendar, we still maintain this tradition of observing two days in the Diaspora.
Nevertheless, when it comes to Chanukah, everybody celebrates it for eight days, including those in the Diaspora. Some explain that we only add a day to biblical holidays but not to rabbinic ones (such as Chanukah). Others feel that the number eight has special significance vis-a-vis Chanukah. This is either because one of the evil decrees of the Greeks against the Jews banned circumcision, which takes place on the eighth day, or because Chanukah was designed to parallel Sukkot (which at the time of Chanukah’s origin was eight days long even in the Diaspora).
We would like to suggest an additional approach. The Beit Yosef poses a famous question: Why do we celebrate Chanukah for eight days? Since the Jews found enough oil to last for one day, the miracle lasted for only seven days. One of the answers proposed is that had they celebrated seven days, then on the fourth day it would have been impossible to tell who was following Beit Hillel and who was following Beit Shammai. Beit Shammai says that on the first night we light eight candles, and on each succeeding night we decrease the number by one. On the final day of the holiday, only one candle is lit. In contrast, Beit Hillel maintains that on the first night we light one candle, and on each succeeding night we increase the number by one. Thus on the eighth day, eight candles are lit. (This is the current custom.) It follows, then, that if we celebrated only seven days of Chanukah, on the fourth day there would be no discernible difference between those following Beit Hillel and those following Beit Shammai (as both would light four candles). To avoid this problem, Chanukah is eight days and not seven. Similarly, if we were to add a day (as we do on other holidays) and celebrate nine days of Chanukah in the Diaspora, this problem would arise on the fifth night. For this reason we do not add a day in the Diaspora, but rather celebrate Chanukah for eight days everywhere.