The Stardew Valley Fair will run from 9 am on the 16th of Fall until 3 pm later that day. If the player does not go into the event area within that period, they will not be permitted to enter the event, and they will be able to continue with the rest of their day as normal. This can be frustrating if you forget; however, the good news is that these are annual events that do not change from year to year. So if you miss one, you'll be able to attend the next one.