Only creatures that are attacking when the last ability resolves will get +1/+0. That is, activating that ability before attackers have been declared won't give the bonus to any creatures, including ones that attack later in the turn.
2014-11-07
wotc
If, during a player's declare attackers step, a creature is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under that player's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.
2014-11-07
wotc
If there are multiple combat phases in a turn, creatures must attack during each combat phase in which they're able to.
2014-11-07
wotc
Each creature's controller still chooses which player or planeswalker the creature attacks.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Not Legal
Timeless
Not Legal
Gladiator
Not Legal
Pioneer
Not Legal
Modern
Not Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Not Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Not Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Not Legal
Who's Online
Live site activity across collectors, guests, and crawlers.