On the 19th of Summer, Barley will greet you in the morning and announce that the Cow Festival will occur the next day. He asks if you would like to enter a cow for judging. You can enter adult (large) cows but they can't be pregnant.
The next day go to Rose Plaza at 10:00 am. You can see all the other cows that have been entered into the contest. Talk to Barley to start the judging. If your cow has 8 or more hearts, she will be declared the winner of the festival. When the festival is over and you leave the plaza, your cow will be placed back in your barn for you. Don't forget to feed her before you go to bed!
Cow festival winners will start to produce Gold or P-Type milk. If you've left your cow inside most of her life, she'll give Gold milk. If you've left her outside for a total of 25 game days (600 hours) then she'll produce P-Type milk.
If you win you'll earn 10 Animal points.