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Some lands and artifacts have abilities which allow them to be animated, and turned into creatures until end of turn. Examples include Inkmoth Nexus, Blinkmoth Nexus, Azorious Keyrune, Treetop Village and several others. These abilities can be activated at any time, and then those permanents can be used to attack that turn.
At what point during the turn should abilities that animate non-creatures be activated so that they can attack?
You should activate them at some point before the declare attackers step on a turn after they enter the battlefield (unless they have haste).
Here's a breakdown of the declare attackers step:
508.1. First, the active player declares attackers.
508.2. Second, any abilities that triggered on attackers being declared go on the stack.
508.3. Third, the active player gets priority. Players may cast spells and activate abilities.
Clearly, during the declare attackers step, it is too late to activate the abilities (508.3) in order to make them attackers (508.1). However, you are free to do so during your upkeep, first main phase, and beginning of Combat step and it will be eligible to attack.
Note that if you cast a keyrune and turn it into a creature, it cannot attack. This is because permanents that are creatures are unable to attack or use activated abilities with the tap symbol in them unless they have been under their owners control continuously since their most recent turn began (302.6).
It could still be tapped for mana if it is not a creature but as soon as you turn it into a creature it can no longer be tapped to activate abilities the turn it comes into play.
With regards to shortcuts, according to the
A statement such as "I'm ready for combat" or "Declare attackers?" offers to keep passing priority until an opponent has priority in the beginning of combat step. Opponents are assumed to be acting then unless they specify otherwise.
In other words, the following exchanges between Aaron the Attacker and Dave the Defender, officiated by Jerry the Judge, have different outcomes
Aaron: Attacks? long pause
Dave: nods head
Aaron: I activate my keyrune and swing with him for 2.
Dave: Judge!
Jerry: Sorry Aaron, because you passed priority with your "Attacks?" comment, your keyrune was activated too late to be an attacker.
Aaron: Attacks - I activate my keyrune. long pause
Dave: nods head
Aaron: I swing with him for 2.
Dave: I take 2, I'm at 18 life.
Jerry: I'm going to sip this margarita because I'm not needed here. sips margarita
Update
Having discussed the below with several local judges, it has been made clear that the below would actually be a very totalitarian view for a judge to take, and as long as you state clearly your intentions you probably will still get to activate your man land.
The correct time to activate man lands is any point during your turn before the declare attackers step, as has been mentioned in other answers. As an addendum, being extremely clear about where you are in the turn when you are activating them is key in tournament play. Better safe than sorry!
The main parts of the turn relevant to this question are:
some of the sub steps are omitted, as they are not relevant.
When you are playing a game of magic, you are playing with/against another person. This is the key to the answer to this question, as while you are entitled to animate these permanents at any point before the Declare Attackers step, in a real game of magic,
unless the active player explicitly passes priority in their main phase, to have priority in their beginning of combat step. the active player only has priority in their beginning of combat step if their opponent chooses to do something in that step.
this is due to a shortcut listed in the rules, wherein the active player declares "attacks?" or "move to attack" or "combat" or some similar statement. This shortcut, in the literal sense, translates to "Pass priority up until you have priority in the Beginning of Combat step". The result of this shortcut is, if you move into the combat phase, you are automatically giving your opponent priority just before you declare attackers. If your opponent passes priority, the game moves to the Declare Attackers step and you have missed your opportunity to animate.
The latest safe opportunity to animate your man lands, or other permanents that become creatures through activating an ability, is in the First Main Phase. if you leave it until after this point, you are in the hands of your opponent and the judge staff for whether you get to activate the man land. If you cant demonstrate a good reason (for example, to empty some mana from your opponent's mana pool), they will probably not let you activate.
Source for this shortcut: Tournament Procedure Guide - section 4.2, Bullet point 2
A Statement such as "I'm Ready for combat", or "Declare attackers?" offers to keep passing priority until an opponent has priority in the beginning of combat step. Opponents are assumed to be acting then unless they specify otherwise
also covered in these articles
http://www.wizards.com/Magic/Magazine/Article.aspx?x=judge/article/20071109a
https://www.wizards.com/Magic/Magazine/Article.aspx?x=judge/article/20050204a
