Right after a performer has added extra tour dates. At this time, the supply has usually increased more than the demand. Fewer people tend to want to attend matinee shows and weekday shows, so tickets for those shows will be easier to buy, though unlike matinee movie tickets, they probably won't be any cheaper.
FYI:
Movie tickets are cheaper early in the day - between late morning and late afternoon - because theaters want to attract people who haven't eaten lunch or dinner and will fill up on popcorn, candy, and sodas. The amount of money these theatergoers save on matinee tickets pales in comparison to what they spend on food and drinks.
MFS-Useful Link:
SeatGeek. SeatGeek is the web's largest search engine that helps you find the best deals on tickets for concerts, theatre, and sporting events. They look high and low through dozens of ticket sellers, such as Stubhub, ebay, TicketsNow, and many more.