Creature â Human Warrior
If Combat Celebrant hasn't been exerted this turn, you may exert it as it attacks. When you do, untap all other creatures you control and after this phase, there is an additional combat phase. (An exerted creature won't untap during your next untap step.)4/1
Chris Rallis INTERVIEW
You canâ€™t exert a creature unless an effect allows you to do so. Similar effects that â€œtap and freezeâ€ a creature (such as that of Decision Paralysis) donâ€™t exert that creature.
Untapping an attacking creature doesnâ€™t remove it from combat.
Thereâ€™s no main phase between your combat phases, so youâ€™ll have no opportunity to cast spells or activate abilities that could only be cast any time you could cast a sorcery. For example, you wonâ€™t be able to cast another creature or equip Equipment between combats.
If you gain control of another playerâ€™s creature until end of turn and exert it, it will untap during that playerâ€™s untap step.
If you exert multiple Combat Celebrants in one combat phase, youâ€™ll have that many additional combat phases, but all of your creatures are untapped only during the current combat phase. Youâ€™ll need to exert the Combat Celebrants one at a time, in multiple combat phases, to untap your attacking creatures and attack with them in each combat step.
If you exert Combat Celebrant, you get an additional combat phase even if Combat Celebrant doesnâ€™t survive the first combat phase.
If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exertâ€™s effect preventing it from untapping expires without having done anything.
Combat Celebrantâ€™s ability untaps all of your creatures, not just the ones that are attacking.
All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You canâ€™t do so later in combat, and creatures put onto the battlefield attacking canâ€™t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.