I think when you are new is when you experience the most no shows... and as you get more experience in the business your instincts kind of can tell when a guy is bullshitting... also, if someone no shows me without a courtesy call, I just save their number so that I don't have to deal with them again in the future.  Other than that, I am not sure if there is a certain time of the year when it happens most... I think it just kind of happens all year long lol.