1.

Play the Song of Time before the final hour to reset — you keep key items and masks.

2.

The Inverted Song of Time slows the clock to 1/3 speed — use it to complete quests.

3.

Anju & Kafei quest spans all 3 days — it's the most elaborate side quest in Zelda history.

4.

Fierce Deity Mask (final mask) makes Majora trivially easy — collect all 20 non-transformation masks.

5.

Deku Link can fly on flowers — use it to reach the first dungeon.

6.

Giant's Wallet and larger quivers are sold at the Curiosity Shop on Night 2.