Why is this a big day for the entire village?

They are excited to think that one of their own is going to really escape the poverty that their oppressed race has had to deal with. This is a day for everyone to celebrate.

The entire town shared in Kino and Juana's excitement over the pearl.

"It is wonderful the way a little town keeps track of itself and of all its units. If every single man and woman, child and baby, acts and conducts itself in a known pattern and breaks no walls and differs with no one and experiments in no way and is not sick and does not endanger the ease and peace of mind or steady unbroken flow of the town, then that unit can disappear and never be heard of. But let one man step out of the regular thought or the known and trusted pattern, and the nerves of the townspeople ring with nervousness and communication travels over the nerve lines of the town. Then every unit communicates to the whole."