Infinite creature ETBInfinite creature LTBInfinite combat phasesInfinite creature tokens with hasteInfinite death triggersInfinite mana creatures you control can produceInfinite creature sacrifice triggersInfinite untap of creatures you control
How it works
Declare Grub and Combat Celebrant as attackers, choosing to exert Combat Celebrant as it attacks. Grub and Combat Celebrant trigger.
Resolve the Grub trigger, putting a -1/-1 counter on Combat Celebrant and then creating a tapped and attacking token copy of it.
Combat Celebrant dies as a state-based action due to having 0 toughness.
Resolve the Combat Celebrant trigger, causing you to untap all other creatures you control and causing you to get an additional combat phase after this one.
Repeat each combat phase, using the Combat Celebrant token instead.