The Roman Martyrology lists multiple saints for every day of the year…including important feast days such as yesterday’s feast of the Immaculate Conception (Dec 8). I know that on a feria day an optional memorial could be celebrated, but as feasts and solemnities always take precedence, how could you ever celebrate the optional memorial of an obscure saint who is commemorated on Dec 8 or Aug 15? Would a votive mass be possible on another available feria day?
As a votive on a free day.
Also, some saints’ days “move up” in importance in certain dioceses (like their home ones, or in dioceses and orders that have slightly different calendars) or in certain parishes (like ones dedicated to them, or - I think - ones that have relics from them).
But votive Masses are usually how it’s done.