Given the following passage  "When not celebrating Mass but still serving a liturgical function, such as the semiannual Urbi et Orbi papal blessing, some Papal Masses and some events at Ecumenical Councils, cardinal deacons can be recognized by the dalmatics they would don with the simple white mitre (so called mitra simplex).",  answer the following question. Note that the answer is present within the text.  Question: What is the one event that is conditional for the deacons in the event described?
The answer to this question is:
When not celebrating Mass