Laetare means "rejoice" in Latin. The 40 days of Lent are a time for solemnity among Roman Catholics - a time for removing vices and other distractions to facilitate meditative reflection. The wise and wily church recognized that people may need a break to tide some through Lent.
The fourth Sunday was considered a day of relaxation from the normal rigors imposed. A day of hope when Easter is within sight.
rose chasuble (Sunday Gaudete and Laetare)
Traditionally, baptisms, confirmations, and weddings are performed on this day. Otherwise, celebrations were generally discouraged and even banned during the Lenten season of sorrow.