Mobilized District — ruling 1

If you control four or more legendary permanents that are creatures and/or planeswalkers, the last ability of Mobilized District costs no mana to activate. A legendary planeswalker that's also a creature reduces this cost by only {1}.

Mobilized District — ruling 2

If Mobilized District becomes a creature the same turn it enters the battlefield, you can't attack with it or activate its mana ability.

Mobilized District — ruling 3

An ability that turns a land into a creature also sets that creature's power and toughness. If the land was already a creature (for example, if it was the target of Awakening of Vitu-Ghazi), this will overwrite the previous effect that set its power and toughness. Effects that modify its power or toughness will continue to apply no matter when they started to take effect. The same is true for counters that change its power or toughness (such as +1/+1 counters) and effects that switch its power and toughness. For example, if Mobilized District has been made a 0/0 creature with nine +1/+1 counters on it, activating its last ability will turn it into a 12/12 Citizen creature that's still a land.

Mobilized District : règles, fonctionnement et interactions (MTG)

Mobilized District is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Add . : This land becomes a 3/3 Citizen creature with vigilance until end of turn. It's still a land. This ability costs less to activate for each legendary creature and planeswalker you control.

How it works : Mobilized District

Original English text — Wizards of the Coast / Scryfall

#1May 3, 2019· Wizards of the Coast
If you control four or more legendary permanents that are creatures and/or planeswalkers, the last ability of Mobilized District costs no mana to activate. A legendary planeswalker that's also a creature reduces this cost by only .
No other card is named in this ruling.
#2May 3, 2019· Wizards of the Coast
If Mobilized District becomes a creature the same turn it enters the battlefield, you can't attack with it or activate its mana ability.
No other card is named in this ruling.
#3May 3, 2019· Wizards of the Coast
An ability that turns a land into a creature also sets that creature's power and toughness. If the land was already a creature (for example, if it was the target of Awakening of Vitu-Ghazi), this will overwrite the previous effect that set its power and toughness. Effects that modify its power or toughness will continue to apply no matter when they started to take effect. The same is true for counters that change its power or toughness (such as +1/+1 counters) and effects that switch its power and toughness. For example, if Mobilized District has been made a 0/0 creature with nine +1/+1 counters on it, activating its last ability will turn it into a 12/12 Citizen creature that's still a land.
No other card is named in this ruling.