...for the feast of St. Luke tomorrow. (or today if it's October 18th when you read this. )
The feast of this Apostle, Evangelist, and patron of Physicians and artists is an obligatory memorial. So you just don't read the current weekday in the psalter plus a concluding prayer in the saints proper. St Luke has an office of his own--a combination of elements that are just for him in the proper of Saints for Morning Prayer with the psalmody of Sunday Week I, and likewise, unique items for his feastday plus psalmody from the Common of Apostles for Evening Prayer. It's pretty easy to find all this in your breviary--the proper of saints for October 18th tells you just what to do.