If an effect allows a player to perform an action “any time [that player] could mulligan,” the player may perform that action at a time they would declare whether they will take a mulligan. This need not be in the first round of mulligans. Other players may have already made their mulligan declarations by the time the player has the option to perform this action. If the player performs the action, they then declare whether they will take a mulligan.
April 27, 2018 · Dominaria
If a card allows a player to reveal it from their opening hand, the player taking this action does so. The card remains revealed until the first turn begins. Each card may be revealed this way only once.
September 30, 2011 · Innistrad
If a card allows a player to reveal it from his or her opening hand, the player taking this action does so. The card remains revealed until the first turn begins. Each card may be revealed this way only once.
June 17, 2011 · Commander 2011
If a card allows a player to reveal it from his or her opening hand, the player taking this
May 1, 2011 · New Phyrexia
If a card allows a player to reveal it from his or her opening hand, the player taking this action does so. The card remains revealed until the first turn begins. Each card may be revealed this way only once.