What is a vigil?

Vigil. The day or eve before a more or less prominent feast or solemnity. It was observed as a preparation for the following day with special offices and prayers and formerly with a fast, honoring the particular mystery of religion or the saint to be venerated on the feast day. The Church today observes solemn vigils for Christmas, Easter, and Pentecost. Although the number of such solemn vigils has been reduced since the Second Vatican Council, the church sill wants the notion to vigils to be kept alive in the minds of the faithful. Thus "it is fitting that Bible services on the vigils of great feasts, on certain ferial days of Lent and Advent, on Sundays and feastdays, should also have the same structure as the liturgy of the Word at Mass" (Inter Oecumenici, 1964, 38). (Etym. Latin vigilia, from vigil, alert.)