Q. 1. What is the purpose of the dalmatic in the Catholic Church?

A. 1. The dalmatic is the outer liturgical vestment of the deacon. It is worn at Mass and at solemn processions and benedictions, except when these processions and benedictions have a penitential character, as in Advent, during the period from Septuagesima Sunday to Easter, at the blessing of candles and the procession on Candlemas Day, etc.; this is because the dalmatic has been regarded from the earliest times as a festal garment. The dalmatic is also worn by bishops under the chasuble at solemn pontifical Mass, but not at private Masses. Priests are not permitted to wear the dalmatic under the chasuble unless a special papal privilege to this effect has been granted, and then only on those days and occasions for which the permission, has been given.

[Source: Catholic Encyclopedia).