According to the Jewish calendar it is actually a very common occurrence to have the Festivals occur on Shabbat. When that happens Shabbat and the Festival are amalgamated and the Festival becomes an integral part Shabbat. That means that the Laws of Shabbat are kept as usual but the Prayers that are recited reflect the fact that the day is both Shabbat and Festival. Most of the customs of the Festival are kept so, for example, if it is Sukkot we sit in the Sukkah, if it is Pesach we commemorate the Seder Night in the regular way.