We have a saying "the early bird catches the worm" meaning the people who move swiftly and act early have an advantage. I am wondering in this case, maybe she means to be fair to everyone, she doesn't want some people arriving before the announced opening time for the yard sale. People might do that to gain an unfair advantage by being able to look at the full choice of items before others had a chance to see everything, and thus take the best things for themselves. That is my best guest about this situation anyway. Just for fun, some people also say "the early bird catches the worm, but the second mouse gets the cheese" meaning the first mouse has died in the mousetrap so the second one gets to eat the cheese! That is just a joke saying however.