They also apply to Feasts. Sundays, Feasts and Solemnities. They are optional, for those using the OOR as traditional “Vigils”.
You would use this with Sunday readings. Typically in a monastery, Vigils would be any time from after 11 pm to 6 am or so. Before 11 pm, they would be anticipated Vigils.
Some monasteries would do anticipated vigils combined with Compline with two nocturnes, the first being the OOR psalms, followed by the bible reading and responsory (first nocturne), and the psalms of Compline followed by the patristic reading (2nd nocturne). Then on Sundays, feasts and solemnities, the third nocturne (the optional canticles and gospel, and in some lectionaries there’s a third reading, also patristic, or a homily). Then the Te Deum (Sundays, Feasts and Solemnities except Sundays of Lent) and the collect, and usually also the Marian antiphon of Compline. I stayed at a monastery in France that did this.
If said the same day (say after 11 pm or so), then the “second nocturne” would be the patristic reading alone with responsory, without the psalms of Compline, if using the 4-week LOTH.
For most monastic breviaries there are two nocturnes of 3 to 6 psalms each on regular days, and the third nocturne is the OT canticles + gospel + 3d reading or homily on Sundays, Feasts and Solemnities.