These types of things are widely respected and held off limits across almost all underworld platforms.

Church's, Synagogues, Weddings, Funerals - too many civilians/innocents promising to be there. Each one is a witness. Not to mention the chance of killing women and children.

This kind of attempt would only occur in the most extreme of situations.

This idea of a general truce or safe zone in regards to religion and funerals is profiled in The Wire after an attempt on someone's life at church breaks the "Sunday truce" held in effect so people could safely attend church with their family.