Opera tends to be a fairly rigid form, from the narrow range of repertoire embraced by its devotees to the seemingly unbreakable tradition of the evening performance. But in the last several years, Lyric Opera of Chicago has discovered a surprising niche audience that has apparently been patiently waiting for someone to start offering weekday matinees. “The decision to add the extra matinees may have had a Field of Dreams effect on a certain demographic — the one made up of folks who might not feel quite as energetic as they used to when it comes to staying awake through an evening three-hour opera and, possibly, a long drive back to the suburbs around midnight.”