Q: In a July 2021 post you stated: A chasuble is only worn for Mass and for the Passion on Good Friday. A priest or deacon may wear alb and stole for a baptism. According to custom for such services as a baptism outside Mass, the cope is optional for a priest, the dalmatic is optional for a deacon.
Where can I find the reference making a dalmatic optional for a deacon? at what other celebrations may a dalmatic be used? LoH? Benediction? Funeral outside of Mass or other funeral rite?
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A: Check the ritual books one by one.
For example, the Order of Baptism of Children never mentions the dalmatic, in spite of my comment in that post from 2021, but it does permit a presiding deacon to wear a cope (OBC 35).
The Order of Celebrating Matrimony says a presiding deacon may wear a dalmatic (OCM 80).
The General Instruction of the Liturgy of the Hours says deacons may wear dalmatics (no. 255).
For benediction, a deacon wears a cope (HCWEMOM 92).
For a funeral outside Mass a deacon may wear a cope (OCF 182).
I’m not sure why these differences exist—perhaps because the rites were composed over a period of time without a constant eye on diaconal vesture. Or perhaps due to differing traditions. But check each ritual book for guidance