Once you've retrieved it from Skull Kid, playing the song of time warps you back to the beginning of the three day period, allowing you to save your progress and, essentially, take as much time as you need to finish the game. That's where the Ocarina of Time comes in.
So it's up to Link to use those 72 hours to stop Skull Kid's evil plan - but it'll take a lot more than three days to succeed. Skull Kid's plan will come to fruition in three days. Link, of course, gives chase and eventually discovers that Skull Kid, under the influence of Majora's Mask, is planning to cause the moon to crash into the planet's surface - more specifically, the village of Clock Town. While riding through the forest, his ocarina and his horse are stolen by Skull Kid. Link, finally back to his youthful self after defeating Ganon, decides to leave his familiar surroundings in search of adventure. Majora's Mask opens shortly after the completion of Ocarina of Time. As a result, Majora's Mask is about as different as you can possibly get without changing the core elements that make it a Zelda game. The idea was to present a new adventure for Link that kept the same basic gameplay and engine elements as the last Zelda game, Ocarina of Time.
Originally slated to appear on Nintendo's ill-fated Nintendo 64 disk drive add-on, the 64DD, this aside to the main Legend of Zelda storyline eventually made the jump to the N64's standard cartridge format.