A more thorough explanation:

Definition: In Ecclesiastical law, a vigil is the day before any solemn feast.

Example: Christmas Eve is a vigil because it is the day before the solemn feast of Christmas.

Explanation: A vigil is a religious observance that takes place on the day before a significant religious event. It is a time of preparation and reflection before the celebration of the feast day. In the example given, Christmas Eve is a vigil because it is the day before Christmas, which is a significant feast day in the Christian religion. During the vigil, people may attend special services or engage in other religious practices to prepare themselves for the upcoming celebration.