Celebrations OK During Advent

Yes, a Catholic church can have a celebration during the Advent season.

The confusion arises because both Advent and Lent use the liturgical color purple and omit the Gloria at Mass. During Lent, parishes curtail and avoid celebrations due to the penitential nature of the season. People, therefore, assume that the same is true of Advent. However, Advent is not listed as a penitential season (canon 1250), and instead of prohibiting unaccompanied music and altar flowers as during Lent, the Church puts forth only that they should be done in moderation during Advent (GIRM 305).