Historical Rules · 29 September 2017
Rule 603.6a
Official source text from the 29 September 2017 Rules release
Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, “When [this object] enters the battlefield, . . . “ or “Whenever a [type] enters the battlefield, . . .” Each time an event puts one or more permanents onto the battlefield, all permanents on the battlefield (including the newcomers) are checked for any enters-the-battlefield triggers that match the event.