From the New Catechism of the Third Order:

The monthly meeting of the fraternity is the Third Order’s substitute for the community life of convent religious.  The Third Order is a true order, with a rule, a postulantship, a novitiate, a profession, a habit, a divine office and the common exercises of a monthly meeting.  It is at the monthly meeting that each fraternity functions as a brotherhood.

In the monthly meeting, tertiaries receive spiritual instruction and guidance, mutual encouragement and edification, renewed inspiration to observe the rule more faithfully and to grow in the spirit of their order.  It is a periodical renewal of the spirit which must last a whole month.