Getting into a show last minute is always a risky proposition. If you are able to get a space in a fair or festival at the last minute, that means they still have vendor spaces available. This could very likely mean it is not a good show because otherwise, all spaces would be booked well in advance (in some instances it could mean the event had a last minute cancellation, but that is not very common.)
We provide excellent data on 26,000+ events, but we have no control over the quality of each show, it is up to you to evaluate each event and determine if you think it will work for you (i.e. be a profitable show), before you commit your time and money.
This is either the date all bookings will be done or the deadline to receive applications (if they use an application process). In either case, contact the event well in advance (4-6 months) of the posted deadline. If a festival is all booked up before their posted deadline, that is the nature of the business.