If Day of the Dragons leaves the battlefield before its first ability has resolved, its second ability will trigger and you’ll sacrifice any Dragons you already control. Then its first ability will resolve and exile your creatures forever, leaving you with that many Dragon tokens.

Token creatures will count to determine how many Dragon tokens you create, but they won’t be returned to the battlefield when Day of the Dragons leaves the battlefield.

Auras attached to the exiled creatures will be put into their owners’ graveyards. Equipment attached to the exiled creatures will become unattached and remain on the battlefield. Any counters on the exiled creatures will cease to exist.

All of the nontoken creatures you exiled return to the battlefield when Day of the Dragons leaves the battlefield, even if some or all of the Dragon tokens had already left the battlefield.