Combat Celebrant

Combat Celebrant's ability untaps all of your creatures, not just the ones that are attacking.

If you exert multiple Combat Celebrants in one combat phase, you'll have that many additional combat phases, but all of your creatures are untapped only during the current combat phase. You'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.

Untapping an attacking creature doesn't remove it from combat.

If you gain control of another player's creature until end of turn and exert it, it will untap during that player's untap step.

There's no main phase between your combat phases, so you'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 won't be able to cast another creature or equip Equipment between combats.