Immediately after the beginning of combat step begins, if the game being played is a multiplayer game in which the active player’s opponents don’t all automatically become defending players, the active player chooses one of their opponents. That player becomes the defending player. See rule 507.1.
November 19, 2021 · Innistrad: Crimson Vow
Immediately after the declare attackers step begins, the active player declares attackers. See rule 508.1.
April 27, 2018 · Dominaria
Immediately after the declare blockers step begins, the defending player declares blockers. See rule 509.1.
February 1, 2014 · Born of the Gods
Immediately after blockers have been declared during the declare blockers step, for each attacking creature that’s become blocked by multiple creatures, the active player announces the damage assignment order among the blocking creatures. See rule 509.2.
September 30, 2011 · Innistrad
Immediately after blockers have been declared during the declare blockers step, for each attacking creature that's become blocked by multiple creatures, the active player announces the damage assignment order among the blocking creatures. See rule 509.2.
June 17, 2011 · Commander 2011
Immediately after blockers have been declared during the declare blockers step, for each
April 1, 2011 · Duel Decks: Knights vs. Dragons
Immediately after blockers have been declared during the declare blockers step, for each attacking creature that's become blocked by multiple creatures, the active player announces the damage assignment order among the blocking creatures. See rule 509.2.
February 4, 2011 · Mirrodin Besieged
Immediately after blockers have been declared during the declare blockers step, for each attacking creature that’s become blocked by multiple creatures, the active player announces the damage assignment order among the blocking creatures. See rule 509.2.
October 1, 2010 · Scars of Mirrodin
Immediately after blockers have been declared during the declare blockers step, for each
June 18, 2010 · Archenemy
Immediately after blockers have been declared during the declare blockers step, for each attacking creature that's become blocked by multiple creatures, the active player announces the damage assignment order among the blocking creatures. See rule 509.2.
February 1, 2010 · Worldwake
Immediately after the active player has announced damage assignment orders (if necessary) during the declare blockers step, for each creature that's blocking multiple creatures, the defending player announces the damage assignment order among the attacking creatures. See rule 509.3.