It says on the official site I look on:
Walk from Main Pathway to Vesta's Farm
» 6:00 am to 10:00 am, not Winter season, Friday, sunny day
Out in front of the shop Marlin, Celia, and Vesta are glad to see you. They just receive a notice from Skye, staying that he'll be there at midnight to get their crops! Celia doesn't feel there's much the 3 of them can do but maybe you can lend them your power? Tell them you can (りょーかい！) and Vesta tells you to return to their house between 10:00 pm and midnight.
At the correct time, meet up with the group inside Vesta's house. Now you are all together to fight for the important crops. Outside Celia thinks you all need to think of a strategy, so Marlin figures you'll all split up and hide. You and Vesta take the southern field while Celia and Marlin take the northern one.
The two of you discover Skye in his hiding place in the field. Vesta tells him he won't be taking their crops, but Skye puts on his romantic charms and tries to tell her that a beautiful face is spoiled with anger. That doesn't work on ol' Vesta! Tell him to stop stealing again (もう盗みやめて!!) but he just laughs at you. Are you worried about him? Well he's not ready for that yet and unleashes his Maiden Chick Beam on the both of you! Now that you and Vesta are frozen, Skye decides his work for tonight is done and tries to leave, but ends up running into Celia and Marlin. Skye tries his ways on Celia and is cut off by Marlin. Skye isn't interested in pursuing men. He freezes Celia with his Maiden Chick Beam and leaves Marlin to deal with a stuck girl.
After Skye escapes you gather back in front of the shop. Marlin and Celia are disappointed they didn't catch him, but Vesta cheers them up
( All information from fogu.com )
I tried it, and now its fall. I kept trying from spring to summer.
