The power of the attacking creature is checked only when the ability triggers. Once it triggers, Cavalcade of Calamity will deal 1 damage to the appropriate player or planeswalker even if the creature’s power changes or the creature leaves the battlefield before the ability resolves.

A creature with power 0 or less can attack, as long as no rule or effect prohibits it from doing so. Many creatures with 0 power have defender, but not all do.

If multiple creatures you control with power 1 or less attack, Cavalcade of Calamity’s ability will trigger for each of those creatures. Each ability will cause Cavalcade of Calamity to deal 1 damage to the appropriate player or planeswalker.