There is no restriction to praying an optional memorial during this period in Advent. The usual rubrics apply for an optional memorial. During the last week of Advent (Dec. 17-24) all memorials are optional as it it is a privileged time. This also applies to all of Lent.
During a privileged time, the manner to celebrate an optional memorial is thus: the entire Office for that day of the last week of Advent (or all of Lent) is prayed. After the collect, there is no concluding rite. Instead you say/chant the Gospel canticle antiphon for the saint being commemorated, and then you say the collect of the saint, and then the concluding rites.