Days designated for giving special honor to God, to the Saviour, to saints, and to holy things. Some are fixed festivals, each having a certain day of the year assigned to it. Others are movable, occurring earlier or later in different years. Festivals are also divided into holy days of obligation and ordinary festivals: on the former the faithful are required to assist at Mass and abstain from unnecessary servile work; on ordinary festivals the Church merely observes the feast in her Office and Mass. Festivals are also arranged in three classes, according to their importance: doubles, semidoubles, and simples. Certain days when the Church does not honor any saint are called ferials (feria, a weekday). Some great festivals have octaves, which extend the solemnity of the feast through eight days; and some have vigils, in which the observance of the feast is begun on the preceding day.