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.
- Beginning Phase
- Main Phase
- Combat Phase
- Beginning of Combat step
- Declare Attackers step
- Declare Blockers step
- Damage step
- End of Combat step
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
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