Complex sentence: The event began with a Centennial Mass celebrated by Archbishop Timothy Dolan, after which Dolan joined alumni and friends of the school at a reception featuring student vocal and ballroom dance performances, a silent auction and raffles.
Split into simpler sentences: The event began with a Centennial Mass celebrated by Archbishop Timothy Dolan. Immediately following, Archbishop Dolan joined alumni and friends of the school at a reception featuring student vocal and ballroom dance performances, a silent auction and raffles.