When Day of the Dragons enters the battlefield, exile all creatures you control. Then create that many 5/5 red Dragon creature tokens with flying.
When Day of the Dragons leaves the battlefield, sacrifice all Dragons you control. Then return the exiled cards to the battlefield under your control.
11/17/2017 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.
11/17/2017 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.
11/17/2017 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.
11/17/2017 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.