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In page Gaudete Sunday:

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The season of Advent originated as a fast of 40 days in preparation for Christmas, commencing on the day after the feast of Saint Martin (11 November), whence it was often called Saint Martin's Lent, a name by which it was known as early as the fifth century. In the ninth century, the duration of Advent was reduced to four weeks (a period starting four Sundays before Christmas), and Advent preserved most of the characteristics of a penitential season, which made it a kind of counterpart to Lent. Gaudete Sunday is a counterpart to Laetare Sunday, and provides a similar break about midway through a season which is otherwise of a penitential character, and signifies the nearness of the Lord's coming.[1]