The Great Fairy Mask is in the hands of the Great Fairy of Magic in North Clock Town. The method to obtain it is exactly the same one Link does in Deku form at the start of the game (when he is rewarded with magic power instead): Upon visiting the Great Fairy, Link will find that Skull Kid has shattered her into many Stray Fairies. He must find and bring back the missing one. During the day, the Stray Fairy can be found in the Laundry Pool. During the night, the Stray Fairy will be found in East Clock Town. Once Link takes the Stray Fairy back to the Great Fairy, she will give him the Great Fairy Mask so he can locate Stray Fairies easily in each dungeon and return them to their respective Great Fairies.
When the mask is worn, the hair on it will begin shimmering if there is a Stray Fairy in the room, and will also cause any Stray Fairies in that room to fly towards the wearer. If the Stray Fairy is trapped, as inside a bubble, it must be freed before it can approach.
This is one of the only four dynamic masks in the game (along with the Circus Leader's Mask, the Blast Mask, and the Bunny Hood), as the hair will move when Link runs and shimmer when there are stray fairies nearby.
↑"Young one! Please hear my plea! I have been broken and shattered to pieces by the masked Skull Kid. Please find the one Stray Fairy lost in town, and bring her to this Fairy Fountain." — Great Fairy (Majora's Mask)
↑"Please, hear my plea! The masked Skull Kid has broken me apart and scattered my pieces! Please find a way to return me to the Fairy Fountain in North Clock Town." — Stray Fairy (Majora's Mask)
↑"Kind young one... Scattered about the four temples of this land are Stray Fairies like myself. Please, somehow find the Stray Fairies and return them to their fountains. Surely, they too will add to your strength. Allow me to grant you something good so that the Stray Fairies will not fear you." — Great Fairy (Majora's Mask)