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Insert from the Strategy Guide: "After you've acquired the Goron Mask by helping out Darmani's ghost, and been certified to carry Powder Kegs by the BigGoron in the Goron Village, you can freely access Romani Ranch during all three days. With the Powder Keg certification, you can purchase Powder Kegs from the Goron found in the Trading Post in Clock Town when you wear the Goron Mask. After you have a Powder Keg, you can clear the passage leading to the ranch. Just remember to stand clear after the barrel's beenlit and set down by the boulder."

You can also wait for the third day, by then the digger would have cleared out the boulder. But by then it is too late to do anything.