The Torah commands a Pesakh observance of seven days. This is followed by Reform Jews and those who live in Israel. Conservative and Orthodox Jews outside of Israel celebrate Pesakh for eight days. Around the seventh century BCE, people were notified of a holiday's beginning by means of an elaborate network of mountaintop bonfires. To guard against the possibility of error, an extra day was added to many of the holidays. By the time a dependable calendar came into existence, the additional day was so deeply engrained, the talmudic sages made the practice halakha (law).
--excerpted from The Jewish Home: A Guide for Jewish Living (URJ Press, New York)